Polymeric materials for electrochemical cells and ion separation processes

ABSTRACT

Polymers of intrinsic microporosity are provided herein. Disclosed polymers of intrinsic microporosity include modified polymers of intrinsic microporosity that include negatively charged sites or crosslinking between monomer units. Systems making use of polymers of intrinsic microporosity and modified polymers of intrinsic microporosity are also described, such as electrochemical cells and ion separation systems. Methods for making and using polymers of intrinsic microporosity and modified polymers of intrinsic microporosity are also disclosed.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Nos.62/142,934, filed Apr. 3, 2015, 62/194,138, filed Jul. 17, 2015, and62/307,309 filed Mar. 11, 2016, each of which is incorporated in itsentirety herein for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with government support under Contract No.DE-AC02-05CH11231 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

Membranes (or separators) are critical for ionic conduction andelectronic isolation in many electrochemical devices. For cellarchitectures that utilize redox-active species that are dissolved,dispersed, or suspended in electrolyte, including fuel cells (FCs),redox flow batteries (RFBs), and conversion reaction electrodes, it isalso of value that the membrane prevent active material crossover thatwould otherwise contribute to device shorting, electrode fouling, orirrevocable loss in capacity. Unfortunately, commercial batteryseparators, which feature shape-persistent mesopores, are freelypermeable to most active materials used in RFBs and electrolyte solubleintermediates formed in conversion reaction electrodes. Alternativemembrane separators have thus far relied heavily on variants of aqueoussingle-ion conductors, e.g., Nafion, which may ultimately restrict theuse of certain types of flowable electrodes. Despite the wideavailability of porous materials that might serve effectively asmembrane components, including zeolites, metal-organic frameworks,covalent organic frameworks, carbon nanotubes, cyclic peptide nanotubes,and microporous polymers, rational design rules for achievingion-selective transport via sieving in battery membranes have not beenestablished.

BRIEF SUMMARY OF THE INVENTION

The present description provides separator systems that are useful in avariety of electrochemical cells, including lithium-sulfur batteries andredox-flow batteries, as well as in ion-separation processes. Thedisclosed separator systems make use of polymers of intrinsicmicroporosity to selectively restrict passage of particular ions throughthe separator. Due to their wide diversity and structural properties,polymers of intrinsic microporosity may be advantageous for separatorsystems. Polymers of intrinsic microporosity may advantageously beselected based on their inherent pore size to selectively restrict largeions by size sieving, while allowing smaller ions to transport throughthe pores. Additionally or alternatively, polymers of intrinsicmicroporosity may be modified to aid in the restriction of iontransport, such as by imparting negative charges to the polymerstructure, which may further provide an electrostatic restriction onanionic species, or by crosslinking the polymer, which may provide forfurther improved size sieving properties.

In some embodiments, polymers of intrinsic microporosity are provided.In some embodiments, polymers of intrinsic microporosity may bemodified. A modified polymer of intrinsic microporosity of someembodiments comprises a polymer of intrinsic microporosity having aplurality of repeat units, wherein at least one of the repeat unitsincludes one or more negative charges. A modified polymer of intrinsicmicroporosity of some embodiments comprises a polymer of intrinsicmicroporosity having a plurality of repeat units, wherein one or morenon-adjacent repeat units are crosslinked. A modified polymer ofintrinsic microporosity of some embodiments comprises a polymer ofintrinsic microporosity having a plurality of repeat units, wherein atleast one of the repeat units includes one or more negative charges, andwherein one or more non-adjacent repeat units are crosslinked. Forexample, in some embodiments, the non-adjacent repeat units correspondto different polymer chains.

Polymers of intrinsic microporosity of some embodiments may further beprovided in contact with a support membrane. Inclusion of a supportmembrane may be useful with polymers of intrinsic microporosity toprovide additional features or functionality to the polymers ofintrinsic microporosity, such as additional structural strength, a shapeor form template, or safety features, for example. Additionally,inclusion of a support membrane with a polymer of intrinsicmicroporosity may facilitate use of the polymer of intrinsicmicroporosity as a separator in an electrochemical cell or as separationmembrane in an ion selective separation process.

In some embodiments, the present invention provides methods of makingmodified polymers of intrinsic microporosity, such as the modifiedpolymers of intrinsic microporosity described above. In one embodiment,a method of making a modified polymer of intrinsic microporositycomprises forming a reaction mixture comprising a polymer of intrinsicmicroporosity and a reducing agent under conditions sufficient to formthe modified polymer of intrinsic microporosity. In embodiments,reaction of a polymer of intrinsic microporosity with a reducing agentresults in one or more of the repeat units of the polymer of intrinsicmicroporosity being modified to include a negative charge, such as byway of a reduction reaction with the reducing agent. In one embodiment,a method of making a modified polymer of intrinsic microporositycomprises forming a reaction mixture comprising a polymer of intrinsicmicroporosity and a nucleophile under conditions sufficient to form themodified polymer of intrinsic microporosity. In embodiments, reaction ofa polymer of intrinsic microporosity with a nucleophile results in oneor more of the repeat units of the polymer of intrinsic microporositybeing modified to include a negative charge, such as by way of anucleophilic addition reaction with the nucleophile. In one embodiment,a method of making a modified polymer of intrinsic microporositycomprises forming a reaction mixture comprising a polymer of intrinsicmicroporosity and a crosslinking agent under conditions sufficient toform the modified polymer of intrinsic microporosity. In someembodiments, reaction of a polymer of intrinsic microporosity with or bythe crosslinking agent results in covalent linkages being formed betweennon-adjacent repeat units. For example, in some embodiments, reaction ofa polymer of intrinsic microporosity with or by the crosslinking agentresults in covalent linkages being formed between different polymerchains.

In some embodiments, the present invention provides electrochemicalcells. In one embodiment, an electrochemical cell comprises an anode, ananode electrolyte in contact with the anode, a separator in contact withthe anode electrolyte, wherein the separator comprises a polymer ofintrinsic microporosity, a cathode electrolyte in contact with theseparator, and a cathode in contact with the cathode electrolyte.Optionally, the polymer of intrinsic microporosity is a modified polymerof intrinsic microporosity, such as the modified polymers of intrinsicmicroporosity described above. Optionally, the polymer of intrinsicmicroporosity is an unmodified polymer of intrinsic microporosity.

In some embodiments, the present invention provides methods of selectiveion transport. In one embodiment, a method of selective ion transportcomprises contacting a first side of a separator with a first ionicsolution; and contacting a second side of the separator with a secondionic solution, wherein the separator comprises a polymer of intrinsicmicroporosity. Optionally, the polymer of intrinsic microporosity is amodified polymer of intrinsic microporosity, such as the modifiedpolymers of intrinsic microporosity described above. In embodiments, thefirst ionic solution comprises a first ionic species and the separatorallows transport of the first ionic species between the first ionicsolution and the second ionic solution through the separator. Inembodiments, the second ionic solution comprises a second ionic speciesand the separator provides a size selective restriction on transport ofthe second ionic species from the second ionic solution to the firstionic solution through the separator. Optionally, the separator providesan electrostatic restriction on transport of the second ionic speciesthrough the separator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A provides a schematic illustration of a polymer of intrinsicmicroporosity in accordance with some embodiments. FIG. 1B, FIG. 1C, andFIG. 1D provide a schematic illustrations of modified polymers ofintrinsic microporosity in accordance with some embodiments.

FIG. 2 provides details of methods of making modified polymers ofintrinsic microporosity in accordance with some embodiments.

FIG. 3 provides a schematic illustration of an electrochemical cell inaccordance with some embodiments.

FIG. 4A provides a schematic illustration of a system embodiment for usein a method of selective ion transport. FIG. 4B provides an overview ofa method embodiment for selective ion transport.

FIG. 5 provides a photograph of a film comprising a polymer of intrinsicmicroporosity (top left), the chemical structure of a polymer ofintrinsic microporosity (bottom left), and a schematic illustration of amolecular model of the polymer of intrinsic microporosity (right).

FIG. 6A provides schematic illustrations of molecular models for avariety of polysulfides and lithium bis(trifluoromethanesulfonyl)imide.FIG. 6B provides data showing a radius of gyration for a variety ofcompositions of polysulfides and lithiumbis(trifluoromethanesulfonyl)imide. FIG. 6C provides data showing poresize distributions for two materials.

FIG. 7A provides data showing ambient temperature ionic conductivity fortwo materials. FIG. 7B provides data showing time evolution ofconcentration of two crossover tests and a photograph of the crossovertest cell configuration.

FIG. 8A provides data showing volumetric energy density as a function ofcycle number for three systems. FIG. 8B provides data showing charge anddischarge rate performance of an example system.

FIG. 9 provides a cross-sectional scanning electron micrograph image ofa membrane comprising a polymer of intrinsic microporosity.

FIG. 10 provides data corresponding to a calibration curve of currentvs. concentration obtained via square wave voltammetry.

FIG. 11 provides data corresponding to a calibration curve of currentvs. concentration obtained via square wave voltammetry.

FIG. 12 provides data showing electrochemical impedance spectra of twosystems.

FIG. 13A provides data showing volumetric energy density of differentbatteries as a function of cycle number for battery systems including aseparator embodiment comprising a polymer of intrinsic microporosity.FIG. 13B provides data showing volumetric energy density of differentbatteries as a function of cycle number for battery systems including aCelgard separator.

FIG. 14A provides data showing Coulombic efficiencies of differentbatteries as a function of cycle number for battery systems including aseparator embodiment comprising a polymer of intrinsic microporosity.FIG. 14B provides data showing Coulombic efficiencies of differentbatteries as a function of cycle number for battery systems including aCelgard separator.

FIG. 15A provides data showing discharge and charge profiles for batterysystems including a separator comprising a polymer of intrinsicmicroporosity. FIG. 15B provides data showing discharge and chargeprofiles for battery systems including a separator comprising a polymerof intrinsic microporosity. FIG. 15C provides data showing discharge andcharge profiles for battery systems including a conventional separator.

FIG. 16 provides data showing Coulombic efficiency of a batteryincluding a separator comprising a polymer of intrinsic microporosity asa function of cycle number.

FIG. 17 provides photographs of a polymer of intrinsic microporosity(left) and a modified polymer of intrinsic microporosity (right), alongwith chemical structures of the polymers.

FIG. 18A provides a photograph and configuration of a test cell. FIG.18B provides data showing the effective rate of diffusion across amembrane comprising a polymer of intrinsic microporosity for differentcell concentrations.

FIG. 19A provides a chemical structure of a polymer of intrinsicmicroporosity and a chemical structure of a modified polymer ofintrinsic microporosity. FIG. 19B provides data showing a portion ofobserved NMR spectra. FIG. 19C provides data showing a portion ofobserved and calculated mass spectra. FIG. 19D provides data showing aportion of observed and calculated mass spectra.

FIG. 20A provides data showing the time evolution of Fourier Transforminfrared spectra during modification of a polymer of intrinsicmicroporosity. FIG. 20B provides data showing the time-evolution of peakintensities during modification of a polymer of intrinsic microporosity.FIG. 20C provides pore size distribution data for a polymer of intrinsicmicroporosity and a modified polymer of intrinsic microporosity.

FIG. 21A provides a calibration plot of log(current) vs.log(concentration) and a fit to the data. FIG. 21B provides residualsfrom FIG. 21A, showing that the deviations from the fit are random. FIG.21C provides the calibration plot of FIG. 21A on linear axes.

FIG. 22A provides different proposed chemical structures and conformersfor modified polymers of intrinsic microporosity. FIG. 22B provides datashowing a portion of observed NMR spectra. FIG. 22C provides datashowing a portion of observed NMR spectra.

FIG. 23A provides data showing a portion of observed NMR spectra forthree different temperatures. FIG. 23B provides data showing a portionof observed NMR spectra for three different temperatures.

FIG. 24 provides a 2D NMR spectrum.

FIG. 25 provides data showing an electrospray ionization mass spectrum.

FIG. 26 provides data showing an expanded view of the electrosprayionization mass spectrum of FIG. 25.

FIG. 27 provides a Fourier Transform Infrared spectrum of a modifiedpolymer of intrinsic microporosity.

FIG. 28 data showing a normalized intensity of a portion of FourierTransform Infrared spectra for two different conditions.

FIG. 29A provides data showing adsorption and desorption isotherms for apolymer of intrinsic microporosity and a modified polymer of intrinsicmicroporosity. FIG. 29B provides data showing surface area analysis fora polymer of intrinsic microporosity and a modified polymer of intrinsicmicroporosity. FIG. 29C provides data showing simulated and observedisotherms for a polymer of intrinsic microporosity and a modifiedpolymer of intrinsic microporosity.

FIG. 30 provides a schematic illustration of a modified polymer ofintrinsic microporosity membrane including identification of a chemicalstructure of a charged moiety and a photograph of an example membrane.

FIG. 31 provides example monomer structures for polymers of intrinsicmicroporosity and their calculated reduction potentials.

FIG. 32A provides a cyclic voltammogram of an example polymer ofintrinsic microporosity. FIG. 32B ultraviolet/visible spectra of apolymer of intrinsic microporosity and a modified polymer of intrinsicmicroporosity.

FIG. 33A provides data showing time-evolution of concentration of apolysulfide in a solution where the polysulfide is transported acrossdifferent membranes. FIG. 33B provides data showing time-evolution ofconcentration of a polysulfide in a solution where the polysulfide istransported across different membranes.

FIG. 34 provides discharge capacity vs. cycle number for two differentelectrochemical cells.

DETAILED DESCRIPTION OF THE INVENTION I. GENERAL

The present invention relates generally to unmodified and modifiedpolymers of intrinsic microporosity and techniques for making andmodifying polymers of intrinsic microporosity, such as to impartparticular characteristics to the polymers, such as useful pore sizes,crosslinking character, and charge structure. For example, in someembodiments, polymers of intrinsic microporosity may be modified throughreactions with reducing agents and/or nucleophiles to modify thepolymers of intrinsic microporosity in a way that provides negativecharges to or throughout the polymers. Additionally or alternatively, insome embodiments, polymers of intrinsic microporosity may be modified bycrosslinking. These modifications may enhance the utility of themodified polymers of intrinsic microporosity for a variety of solutionphase applications.

Inclusion of negative charges may be useful, in some embodiments, forimproving the functionality of a polymer of intrinsic microporosity asan ion separation structure or selectively permeable membrane. Forexample, the unmodified and modified polymers of intrinsic microporositymay be used as electrochemical cell separators or ion separationmembranes, and the presence of the negative charges in the modifiedpolymers of intrinsic microporosity may impact, through electrostaticeffects, which sizes and types of ions in a solution are permitted topass through the separators or membranes and which sizes and types ofions in a solution are restricted from passing through the separators ormembranes.

Similarly, the presence and/or extent of crosslinking of modifiedpolymers of intrinsic microporosity may enhance the functionality of apolymer of intrinsic microporosity as an ion separation structure orselectively permeable membrane. For example, modified polymers ofintrinsic microporosity that are crosslinked may also be used aselectrochemical cell separators or ion separation membranes, and thepresence and/or extent of crosslinking may impact, through size sievingeffects, which sizes and types of ions in a solution are permitted topass through the separators or membranes and which sizes and types ofions in a solution are restricted from passing through the separators ormembranes.

In some embodiments, unmodified polymers of intrinsic microporosity areuseful as an ion separation structure or selectively permeable membrane,such as in an electrochemical cell separator or ion separation membrane.Selection of a particular structure for an unmodified polymer ofintrinsic microporosity may, in some embodiments, determine the size ofthe pores of the polymer which may impact the size sieving effects ofthe polymer for permitting/restricting particular sizes and types ofions in solution from passing through the separators or membranes.

In some embodiments, these characteristics make polymers of intrinsicmicroporosity and modified polymers of intrinsic microporosity usefulfor specific electrochemical cell systems, such as lithium-sulfurbattery systems, where undesirable reactions with the electrolytes canresult in irreversible loss of capacity. For example, although the fullycharged cathode material (S₈) and the fully discharged cathode material(Li₂S) may be generally insoluble in most electrolytes, theintermediates (i.e., Li₂S₂, Li₁S₃, Li₂S₄, Li₂S₆, Li₂S₈) may have highelectrolyte solubility. If the intermediates are able to pass from thecathode electrolyte to the anode electrolyte and contact the lithiumcontaining anode, they may react with the anode and result in apermanent loss of the active sulfur cathode material. Advantageously,the unmodified and modified polymers of intrinsic microporosity may beable to stop and/or reduce the rate at which the intermediates may crossfrom the cathode electrolyte to the anode electrolyte.

II. DEFINITIONS

“Polymer” refers to a molecule composed of repeating structural units,referred to herein as monomers or repeat units, connected by covalentchemical bonds or the polymerization product of one or more monomers.Polymers may be characterized by a high molecular weight, such as amolecular weight greater than 100 atomic mass units (amu), greater than500 amu, greater than 1000 amu, greater than 10000 amu or greater than100000 amu. In some embodiments, a polymer may be characterized by amolecular weight provided in g/mol or kg/mol, such as a molecular weightof about 200 kg/mol or about 80 kg/mol. The term polymer includeshomopolymers, or polymers consisting essentially of a single repeatingmonomer subunit. The term polymer also includes copolymers, which areformed when two or more different types of monomers are linked in thesame polymer. Copolymers may comprise two or more monomer subunits, andmay include random, block, alternating, segmented, grafted, tapered andother copolymers. Useful polymers include organic polymers that may bein amorphous, semi-amorphous, crystalline or partially crystallinestates. Crosslinked polymers having monomer units that are linked toother polymer molecules or other parts of the same polymer molecule areuseful for some applications.

“Repeat unit” refers to a part of a polymer that represents a repetitivestructure of the polymer chain, the repetition of which would make upthe complete polymer chain with the exception of end groupscorresponding to terminal ends of the polymer chain. In someembodiments, a repeat unit may also be referred to herein as a monomer.Repeat units may be identified in a polymer structure by brackets orparentheses and include a subscript n, which represents the degree ofpolymerization. In some embodiments, values for subscript n includeintegers selected from, for example, 10 to 1000, 50 to 900, 100 to 800,or 200 to 500. In some embodiments, subscript n is an integer more than1000. It will be appreciated that a value for subscript n in a polymermay not be explicitly provided, consistent with use by skilled artisansin the field of polymers. In the following polymer structure, RUcorresponds to a repeat unit of the polymer:

RU_(n).

“Microporosity” refers to a characteristic of a material describing theinclusion of voids, channels, openings, recessed regions, etc., alsoreferred to herein as micropores, in the body of material. Optionally,the micropores have a cross sectional dimension of about 2 nm or less.In some embodiments, micropores may have a cross sectional dimension ofabout 1.7 nm or less, 1.5 nm or less, 1.2 nm or less, 1 nm or less, or0.8 nm or less. Optionally, micropores may have cross sectionaldimensions selected from the range of 0.5 nm to 2 nm, selected from therange of 0.5 nm to 1.2 nm, or selected from the range of 1.2 nm to 1.7nm. The inclusion of micropores in a material may allow for othermaterials, such as gases, liquids, ions, etc., to pass through themicropores.

“Intrinsic microporosity” refers to a continuous network ofinterconnected voids in a material formed as a direct consequence of theshape and rigidity of the components of the material. Intrinsicmicroporosity is achieved in some polymers by the polymers possessingindividual structural units that are rigid and that may be orientedrelative to one another in such a way that the structural units align toform an opening or pore. Additionally or alternatively, a polymerpossessing intrinsic microporosity may have a structure that exhibitsfrustrated packing. Frustrated packing of a polymer may occur when apolymer molecule contacts itself or other like polymer molecules and therigidity of the molecule(s) causes the molecule(s) to lie in aconfiguration where spaces between the molecule(s) are created. Suchspaces may correspond to micropores in a film or membrane made of thepolymer molecules, for example.

“Polymer of intrinsic microporosity” refers to a polymer that exhibitsmicroporosity due to the shape and rigidity of the molecular structureof the repeat units within the polymer, where the repeat units may alignrelative to one another such that spaces or openings are generated alongthe polymer chain. Additionally or alternatively, the repeat units mayalign in an aggregate of the polymer in a way that frustrates packing ofthe polymer molecules in the aggregate such that spaces or openings aregenerated between different polymer molecules and/or between segments ofthe same polymer molecule. These spaces within the aggregated polymermay, at least in part, provide the microporosity to such a polymer. Dueto the inclusion of the micropores, some polymers of intrinsicmicroporosity may exhibit high surface areas, such as a surface areaselected from the range of 300 m² g⁻¹ to 1500 m² g⁻¹.

Polymers of intrinsic microporosity of some embodiments include a chainof repeat units (i.e., monomers) where adjacent repeat units arecovalently bonded such that they are non-rotatable with respect oneanother, which may provide a degree of rigidity to the polymer chain.Such a non-rotatable covalent bonding configuration between adjacentrepeat units may, for example, be due to the presence of multiple bondsbetween adjacent repeat units and/or a multi-order bond (e.g., a doublebond) between adjacent repeat units. In some embodiments, adjacentrepeat units in the chain may include portions which may be connected bysingle bonds but are sterically restricted from rotating, for exampledue to the size of the portions connected by single bonds. A stericrestriction on rotation may, in part, provide a degree of rigidity tothe polymer chain, in some embodiments.

Additionally, repeat units of polymers of intrinsic microporosity mayfurther include rigid moieties that exhibit planarity, such as aromaticring containing moieties, heteroaromatic ring containing moieties, andpolycyclic moieties including aromatic or heteroaromatic structures, forexample. In some embodiments, multiple portions of a repeat units, suchas different planar structures, are covalently bonded such that they arenon-rotatable with respect one another, which may provide a degree ofrigidity to the polymer chain. Such a non-rotatable covalent bondingconfiguration between repeat unit portions may, for example, be due tothe presence of a non-rotatable linking group providing covalent bondingbetween the repeat unit portions, such as multiple bonds between repeatunit portions, a multi-order bond (e.g., a double bond) between repeatunit portions, or a linking group including a fused ring structure. Insome embodiments, repeat units may include portions which may beconnected by single bonds but are sterically restricted from rotating,for example due to the size of the portions connected by single bonds. Asteric restriction on rotation may, in part, provide a degree ofrigidity to the polymer chain, in some embodiments.

Additionally, some or all of the repeat units may include a rigidlinking moiety between different planar moieties that function to orientthe different planar moieties in non-coplanar orientations. Such a rigidlinking moiety may include, for example, a non-aromatic cyclic moiety,or a non-aromatic polycyclic moiety, or a combination of an aromatic orheteroaromatic moiety and a non-aromatic moiety. Useful rigid linkingmoieties include bridged ring moieties. Useful rigid linking moietiesinclude cyclic spiro moieties, where two ring structures are provided ina fused configuration where only one atom is shared by the two rings.The inclusion of a rigid linking moiety within a repeat unit thatorients planar moieties unit in non-coplanar orientations may furtherserve to give the repeat unit a structure that results in frustratedpacking of the polymer within an aggregated polymer of intrinsicmicroporosity such that the repeat units arrange in a structure wherespace is provided between the repeat units.

In some embodiments, a polymer of intrinsic microporosity does notpossess a crosslinked network of covalent bonds, and microporosity isprovided to the polymer due to the rigid structure of the repeat units.The term “non-network polymer” may refer to a polymer that does notpossess a crosslinked network of covalent bonds between non-adjacentrepeat units, such as repeat units that may be on different polymermolecules, for example. Some polymers of intrinsic microporosity may bemodified, however, through chemical and/or physical processes togenerate a crosslinked structure that may retain the microporositycharacter, at least in part. For example, some polymers of intrinsicmicroporosity may be crosslinked by exposure to ultraviolet radiationand/or microwave radiation. Additionally or alternatively, some polymersof intrinsic microporosity may be crosslinked by heating. Additionallyor alternatively, some polymers of intrinsic microporosity may becrosslinked by exposure to a crosslinking agent, such as2,6-bis(4-azidobenzylidene)cyclohexanone,2,6-bis(4-azidobenzylidene)-4-methylcyclohexanone,2,6-bis(4-azidobenzylidene)-4-ethylcyclohexanone, or4-azidophenylsulfone. In some embodiments, oxygen may serve as acrosslinking agent.

Example polymers of intrinsic microporosity are described in U.S. Pat.Nos. 7,690,514 and 8,056,732, PCT International Patent Publications WO2005/012397 and WO 2005/113121, and “Polymers of IntrinsicMicroporosity” a review article by Neil B. McKeown (ISRN MaterialsScience, Volume 2012, Article ID 513986), each of which are herebyincorporated by reference.

In some embodiments, a polymer of intrinsic microporosity does notpossess a charged moiety. In some embodiments, repeat units of amodified polymer of intrinsic microporosity possess a charged moiety. Itwill be appreciated that introduction of charged moieties may beachieved, in some embodiments, through chemical modification of apolymer of intrinsic microporosity, such by reacting a polymer ofintrinsic microporosity with a reducing agent and/or a nucleophile.

It will be appreciated that, in some embodiments, polymers of intrinsicmicroporosity may be modified to include any useful chemicalfunctionality.

“Charged moiety” refers to a part or functional group of a compound thatpossesses a positive or negative charge. The positive or negative chargeof a compound may be delocalized or shared by different atoms in thecompound, such as may be indicated by different resonance structuralrepresentations of the compound.

“Reducing agent” and “electron donor” refers to a chemical species thatprovides an electron to another chemical species in a redox reactioninvolving the two species. The chemical species that receives theelectron may be referred to as an “oxidizing agent” or an “electronacceptor.” In some embodiments, a reducing agent refers to an agentcapable of reducing an atom from a higher oxidation state to a loweroxidation state.

Any suitable reducing agent is useful in the method of the presentinvention. For example, reducing agents include, but are not limited to,an alkali metal polysulfide, such as

Li₂S_(m), where subscript m is an integer selected from 2 to 100. Otherreducing agents are known to one of skill in the art, such as those in“Comprehensive Organic Transformations”, 1st edition, Richard C. Larock,VCH Publishers, New York, 1989.

“Oxidation potential” refers to a measure of the energy change needed toremove an electron from an atom or a compound. Oxidation potentials maybe provided as a voltage and may be referenced to a standard hydrogenelectrode (SHE), i.e., a voltage amount greater than or less than thatrequired for the reaction of ½H₂ (gas)→H⁺ (solution)+e⁻.

“Nucleophile” refers to a chemical species that reacts with anotherspecies to form a covalent bond with the other species by providing pairof electrons to form the bond. The species with which a nucleophilereacts may be referred to as an “electrophile.” Generally, a nucleophilewill possess a lone pair of electrons that can participate in the bondforming reaction. In some embodiments, a nucleophile may be referred toas a Lewis base. In some embodiments, a nucleophile may be an anionicspecies. In some embodiments, a nucleophile may react with anelectrophile in a nucleophilic addition reaction where components of thenucleophile and the electrophile are combined in the reaction product.

“Crosslink” refers to a process by which covalent bonds are formedbetween separate polymer molecules or between separate monomer sites onthe same polymer molecule. A “crosslink” may also refer to a covalentbond formed between separate polymer molecules or between separatemonomer sites on the same polymer molecule. A crosslink may also referto a chemical species of one or more atoms that forms covalent bondswith separate polymer molecules or between separate monomer sites on thesame polymer molecule.

“Crosslinking agent” refers to a composition used to facilitate formingcrosslinks between separate polymer molecules or between separatemonomer sites on the same polymer molecule. Some crosslinking agentsmay, for example, be a catalyst that is not covalently incorporated intothe polymer molecule but merely increases a crosslinking rate and/orlowers an energy requirement for a crosslinking reaction. Somecrosslinking agents may be directly incorporated, at least in part,within a covalent link between polymer molecules or between separatemonomer sites on the same polymer molecule. Example crosslinking agentsinclude, but are not limited to,2,6-bis(4-azidobenzylidene)cyclohexanone,2,6-bis(4-azidobenzylidene)-4-methylcyclohexanone,2,6-bis(4-azidobenzylidene)-4-ethylcyclohexanone, and4-azidophenylsulfone. In some embodiments, oxygen may serve as acrosslinking agent.

A crosslinking agent may also refer to the addition of energy to apolymer to form crosslinks between separate polymer molecules or betweenseparate monomer sites on the same polymer molecule. For example, heatmay be a crosslinking agent in some embodiments. Additionally oralternatively, ultraviolet radiation may be a crosslinking agent in someembodiments. Additionally or alternatively, microwave radiation may be acrosslinking agent in some embodiments.

“Electrochemical cell” refers to a device that produces electricalenergy through chemical reactions. Example electrochemical cells includebatteries and fuel cells. Batteries may include solid-state batteries,semi-solid batteries, wet cell batteries, dry cell batteries, flowbatteries, primary batteries, secondary batteries, etc. A battery mayrefer to an assembly of a plurality of individual electrochemical cells,such as arranged in a series configuration. Example electrochemicalcells include an anode, a cathode, a separator between the anode and thecathode, and an electrolyte. Electrochemical cells may further include acurrent collector in electrical contact with an electrode and/or anelectrolyte and may be used, in part, to provide a conductive pathbetween the electrode and a load.

“Anode” refers to an electrode in an electrochemical cell whereoxidation occurs during discharge of the electrochemical cell. In someembodiments, an anode is identified in an electrochemical cell as thenegative electrode, where electrons are emitted during discharge for useby a load. In some embodiments, an anode oxidizes material and releasespositive ions to an electrolyte during discharge.

“Cathode” refers to an electrode in an electrochemical cell wherereduction occurs during discharge of the electrochemical cell. In someembodiments, a cathode is identified in an electrochemical cell as thepositive electrode, where electrons are received during discharge afteruse by a load. In some embodiments, a cathode reduces positive ionsreceived from an electrolyte during discharge.

“Separator” refers to an ion conductive barrier used to separate ananode and a cathode in an electrochemical cell. In some embodiments, aseparator is a porous or semi-permeable membrane that restricts thepassage of certain materials across the membrane. In some embodiments, aseparator provides a physical spacing between the anode and the cathodein an electrochemical cell. In some embodiments, a separator is notelectrically conductive and provides a gap in electrical conductivitybetween the anode and the cathode in an electrochemical cell.

“Electrolyte” refers to an ionically conductive substance or compositionand may include solvents, ionic liquids, metal salts, ions such as metalions or inorganic ions, polymers, ceramics, and other components. Anelectrolyte may be a solid, in some embodiments. An electrolyte may be aliquid, such as a solvent containing dissolved ionic species. Anelectrolyte may be used, in some embodiments, for transporting ionsbetween an anode and a cathode in an electrochemical cell.

“Ionic solution” refers to a solvent including dissolved ionic species.An electrolyte is an example of an ionic solution. Useful solvents forionic solutions include aqueous solvents containing water. Usefulsolvents for ionic solutions include non-aqueous solvents, such asorganic solvents.

“Anode electrolyte” refers to an electrolyte in an electrochemical cellin contact with an anode. An anode electrolyte may also be referred toherein as an “anolyte.” An anode electrolyte may further be in contactwith a separator in an electrochemical cell.

“Cathode electrolyte” refers to an electrolyte in an electrochemicalcell in contact with a cathode. A cathode electrolyte may also bereferred to herein as a “catholyte.” A cathode electrolyte may furtherbe in contact with a separator in an electrochemical cell.

“Membrane” refers to a web of material that extends in lateraldimensions, which may be orthogonal to a thickness dimension of themembrane. In some embodiments, the term “membrane” may be usedinterchangeably herein with the term “film”. Optionally, a membraneseparates two regions in space by the physical materials that make upthe membrane. A membrane may be used as a support or template for othermaterials in order to provide structure and/or stability to the othermaterial, for example. The other material may be attached to one side ofthe membrane, and or may encapsulate all or portions of the membrane.

“Support membrane” refers to a structural film that may providemechanical stability to another material coated onto or otherwiseattached to the film. In some embodiments, a support membrane may beporous or otherwise allow materials, such as ions, gases, or liquids, topass through the support membrane, though any coated or otherwisesupported material may restrict, at least in part, the passage of theions, gases, or liquids.

“Selective ion transport” refers to a process where ions of differentchemical species exhibit different transport rates. For example,selective ion transport may refer to a process where ions of aparticular species are restricted from moving, while ions of anotherspecies may be permitted to move. In some embodiments, selective iontransport may be achieved through use of a semi-permeable membrane, suchas a separator.

“Alkali metal” refers to lithium, sodium, potassium, rubidium, cesium,and francium.

“Aryl” refers to an aromatic ring system having any suitable number ofring atoms and any suitable number of rings. Aryl groups can include anysuitable number of ring atoms, such as, 6, 7, 8, 9, 10, 11, 12, 13, 14,15 or 16 ring atoms, as well as from 6 to 10, 6 to 12, or 6 to 14 ringmembers. Aryl groups can be monocyclic, fused to form bicyclic ortricyclic groups, or linked by a bond to form a biaryl group.Representative aryl groups include phenyl, naphthyl and biphenyl. Otheraryl groups include benzyl, having a methylene linking group. Some arylgroups have from 6 to 12 ring members, such as phenyl, naphthyl orbiphenyl. Other aryl groups have from 6 to 10 ring members, such asphenyl or naphthyl. Some other aryl groups have 6 ring members, such asphenyl. Aryl groups can be substituted or unsubstituted.

“Heteroaryl” refers to a monocyclic or fused bicyclic or tricyclicaromatic ring assembly containing 5 to 16 ring atoms, where from 1 to 5of the ring atoms are a heteroatom such as N, O or S. Additionalheteroatoms can also be useful, including, but not limited to, B, Al, Siand P. The heteroatoms can also be oxidized, such as, but not limitedto, —S(O)— and —S(O)₂—. Heteroaryl groups can include any number of ringatoms, such as, 3 to 6, 4 to 6, 5 to 6, 3 to 8, 4 to 8, 5 to 8, 6 to 8,3 to 9, 3 to 10, 3 to 11, or 3 to 12 ring members. Any suitable numberof heteroatoms can be included in the heteroaryl groups, such as 1, 2,3, 4, or 5, or 1 to 2, 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, 2 to 5, 3to 4, or 3 to 5. Heteroaryl groups can have from 5 to 8 ring members andfrom 1 to 4 heteroatoms, or from 5 to 8 ring members and from 1 to 3heteroatoms, or from 5 to 6 ring members and from 1 to 4 heteroatoms, orfrom 5 to 6 ring members and from 1 to 3 heteroatoms. The heteroarylgroup can include groups such as pyrrole, pyridine, imidazole, pyrazole,triazole, tetrazole, pyrazine, pyrimidine, pyridazine, triazine (1,2,3-,1,2,4- and 1,3,5-isomers), thiophene, furan, thiazole, isothiazole,oxazole, and isoxazole. The heteroaryl groups can also be fused toaromatic ring systems, such as a phenyl ring, to form members including,but not limited to, benzopyrroles such as indole and isoindole,benzopyridines such as quinoline and isoquinoline, benzopyrazine(quinoxaline), benzopyrimidine (quinazoline), benzopyridazines such asphthalazine and cinnoline, benzothiophene, and benzofuran. Otherheteroaryl groups include heteroaryl rings linked by a bond, such asbipyridine. Heteroaryl groups can be substituted or unsubstituted.

III. POLYMERS OF INTRINSIC MICROPOROSITY

As described above, polymers of intrinsic microporosity are useful withsome embodiments of the present invention. For example, polymers ofintrinsic microporosity may be directly useful in systems of theinvention, such as electrochemical cells and ion separation systems.Additionally, polymers of intrinsic microporosity may be useful asstarting materials in methods of the invention, such as methods formaking modified polymers of intrinsic microporosity and methods forselective ion transport.

It will be appreciated that many polymers of intrinsic microporosity maybe useful with some embodiments described herein. Polymers of intrinsicmicroporosity may refer to a class of polymers that exhibitmicroporosity simply due to their chemical structure, which may includerigid (i.e., non-rotatable) moieties, such as ring systems, within themonomers of the polymer. The rigidity of the chemical structure mayresult in the polymer chain having spaces or openings between atoms ofthe polymer in a way that frustrates packing of the polymer chain suchthat an aggregate of the polymer (e.g., in a film) has a continuousnetwork of openings or pores that may extend through all or a portion ofthe aggregate. The pore dimensions may be controlled by selective use ofparticular structural elements in the monomers that provide a degree ofnon-linearity to the chain. For example structural elements of themonomers may orient portions of monomers at an angle relative to oneanother such that the monomer has a bent character. Optionally,different portions of the monomer are planar and are provided in anon-coplanar orientation with respect to one another. For example, alinking structural element between the planar portions may orient theplanar portions at an angle with respect to one another. The bent ornon-coplanar character of a monomer may result in a polymer comprising aplurality of the monomers exhibiting spacings between portions of thepolymer.

Polymers of intrinsic microporosity may include a plurality of repeatunits linked in a chain configuration, and may be represented by astructure,

RU_(n),

where RU refers to a repeat unit and where subscript n corresponds to anumber of repeat units in the polymer molecule, also referred to as thedegree of polymerization. In some embodiments, values for subscript ninclude integers selected from, for example, 10 to 1000, 50 to 900, 100to 800, or 200 to 500. In some embodiments, subscript n is an integermore than 1000. It will be appreciated that, in many instances, a valuefor subscript n may not be directly provided in a chemical structure ofa polymer of intrinsic microporosity, consistent with use in the fieldof polymers.

FIG. 1A provides a schematic illustration of an embodiment of a polymerof intrinsic microporosity 100A. Polymer of intrinsic microporosity 100Ais depicted as including three repeat units 101A, 102A, and 103A. Itwill be appreciated that additional repeat units may be included inpolymer of intrinsic microporosity 100A, but for sake of simplicity ofillustration, these additional repeat units are not illustrated in FIG.1A and broken non-rotatable bonds 107B illustrate connection points toadditional repeat units. Each repeat unit 101A, 102A, and 103A includesa first repeat unit portion 104 and a second repeat unit portion 105. Inthe embodiment illustrated, first repeat unit portion 104 and secondrepeat unit portion 105 are covalently linked by a non-rotatable linker106. Additionally, each of the repeat units are linked by anon-rotatable bond 107A. Non-rotatable bond 107B is depicted in FIG. 1Ato represent covalent bonding to additional repeat units that are notdepicted in FIG. 1A. In some embodiments, first repeat unit portion 104includes one or more planar chemical moieties, such as an aromatic orheteroaromatic moiety. Similarly, second repeat unit portion 105 mayinclude one or more planar chemical moieties, such as an aromatic orheteroaromatic moiety. Inclusion of planar chemical moieties in firstrepeat unit 104 and/or second repeat unit 105 may be advantageous asthese chemical moieties may provide rigidity to the repeat units 101A,102A, and 103A. Repeat units 101A and 102A may be referred to asadjacent repeat units since, for example, there are no repeat unitsbetween repeat units 101A and 102A. Repeat units 102A and 103A may bealso referred to as adjacent repeat units. Repeat units 101A and 103Amay, however, be referred to as non-adjacent repeat units since, forexample, there is at least one repeat unit between repeat units 101A and103A, namely repeat unit 102A. Accordingly, repeat units that aredirectly linked to one another may be referred to herein as “adjacent”and repeat units that are linked to one another through one or moreintermediate repeat units may be referred to herein as “non-adjacent.”

Non-rotatable linker 106 may comprise a moiety that includes a chiralcenter. Non-rotatable linker 106 may comprise a bridged ring moiety,such as a norbornane derivative. Non-rotatable linker 106 may comprise acyclic spiro moiety, such as spirononane derivative, which maycorrespond to two fused five-membered rings that share a single atom.

Non-rotatable bonds 107A and 107B may comprise multiple covalent bondsbetween monomers, such as two individual single bonds, a combination ofa single bond and a double bond, or two individual double bonds, forexample. Such a configuration may arise, for example, by virtue of afused ring structure, such as where a first ring structure on a firstend of a first repeat unit is fused to a second ring structure on asecond end of a second repeat unit through a linking ring structurebetween the repeat units.

A variety of polymers of intrinsic microporosity are useful with someembodiments described herein. Useful polymers of intrinsic microporosityand methods of making polymers of intrinsic microporosity are describedin U.S. Pat. Nos. 7,690,514 and 8,056,732, which are hereby incorporatedby reference. Useful polymers of intrinsic microporosity and methods ofmaking polymers of intrinsic microporosity are also described in PCTInternational Patent Publications WO 2005/012397 and WO 2005/113121,which are hereby incorporated by reference. Useful polymers of intrinsicmicroporosity, the properties of polymers of intrinsic microporosity,and methods of making polymers of intrinsic microporosity are describedin the review article by Neil B. McKeown entitled “Polymers of IntrinsicMicroporosity” (ISRN Materials Science, Volume 2012, Article ID 513986),which is hereby incorporated by reference.

In some embodiments, polymers of intrinsic microporosity may becharacterized by a surface area. In some embodiments, polymers ofintrinsic microporosity may be characterized by gasadsorption/desorption amount and rates, such as for N₂adsorption/desorption, which may allow for determination of theirsurface area, for example. Adsorption isotherms may be determined toallow for determination of a Brunauer, Emmett, and Teller (BET) surfacearea. BET surface areas may allow for comparison of microporositycharacters, for example, between different polymers of intrinsicmicroporosity. For example, a first polymer of intrinsic microporositythat exhibits a smaller BET surface area than a second polymer ofintrinsic microporosity may be characterized as having lessmicroporosity than the second polymer of intrinsic microporosity. Usefulunmodified and modified polymers of intrinsic microporosity include, butare not limited to, those exhibiting a surface area of at least 300m²/g, such as a surface area selected from the range of 200 m²/g to 1000m²/g, or from the range of 250 m²/g to 800 m²/g.

In some embodiments, polymers of intrinsic microporosity may becharacterized by their pore size. In some embodiments, polymers ofintrinsic microporosity may be characterized by microporosity.Microporosity and pore sizes of polymers of intrinsic microporosity maybe characterized by determining the effective rate of diffusion of oneor more gases across a film of the polymer having a known thickness.Microporosity and pore size characteristics of polymers of intrinsicmicroporosity may be probed using positron annihilation lifetimespectroscopy, in some embodiments.

In some embodiments, polymers of intrinsic microporosity may becharacterized by their solubility in organic solvents, such astetrahydrofuran or chloroform. In some embodiments, polymers ofintrinsic microporosity may exhibit high solubility in organic solvents,while other polymers may exhibit low or no solubility in organicsolvents.

In some embodiments, polymers of intrinsic microporosity may becharacterized by their molecular weights. Optionally, size exclusionchromatography may be useful for determining molecular weights ofpolymers of intrinsic microporosity. Optionally, gel permeationchromatography may be useful for determining molecular weights ofpolymers of intrinsic microporosity. Molecular weight determination may,in turn, allow for determination of a degree of polymerization of apolymer of intrinsic microporosity. Example polymers of intrinsicmicroporosity include, but are not limited to, those exhibitingmolecular weights of at least 50 kg/mol, at least 100 kg/mol, at least200 kg/mol, or at least 300 kg/mol. In some embodiments, polymers ofintrinsic microporosity exhibit molecular weights selected from therange of about 50 kg/mol to about 250 kg/mol, or from the range of about80 kg/mol to about 200 kg/mol. Example polymers of intrinsicmicroporosity include, but are not limited to, those exhibiting degreesof polymerization selected from the range of 100 to 1000, from the rangeof 200 to 900, from the range of 300 to 800, from the range of 400 to700, or from the range of 500 to 600.

Chemical structure characterization of polymers of intrinsicmicroporosity may be accomplished using a variety of techniques. Suchcharacterizations may also allow for determination of modifications anddegrees of modifications to polymers of intrinsic microporosity. Forexample, ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy may beuseful. Example NMR spectra of embodiments of polymers of intrinsicmicroporosity are depicted in FIGS. 19B, 22B, 22C, 23A-23B, and 25. Insome embodiments, infrared spectroscopy may also be useful. Exampleinfrared spectra of embodiments of polymers of intrinsic microporosityare depicted in FIGS. 20A and 27. Additionally or alternatively,ionization mass spectrometry, such as electrospray ionization massspectrometry, may also be useful for identifying structural moietieswithin a polymer of intrinsic microporosity. Example mass spectra ofembodiments of polymers of intrinsic microporosity are depicted in FIGS.19C-19D, 25, and 26.

Other characterization techniques known to the skilled artisan may beuseful for characterizing unmodified polymers of intrinsic microporosityand modified polymers of intrinsic microporosity. For example, in someembodiments, polymers of intrinsic microporosity may be characterized bytheir ultraviolet and/or visible absorption spectra. As another example,a modified polymer of intrinsic microporosity may be characterized by anextent, density, or degree of crosslinking, such as by use of knownstandard techniques that evaluate how much a crosslinked polymer swellsin a particular solvent at a particular temperature. Example standardsinclude ASTM D2765 and ASTM F2214.

Useful polymers of intrinsic microporosity may be formed throughpolymerization reactions of suitable monomers or monomer sub-portions.For example, in some embodiments, a polymer of intrinsic microporosityis formed via a step-growth polymerization reaction. The followingprovides an illustration of a polymerization reaction for forming apolymer of intrinsic microporosity of one embodiment:

In some embodiments, the present invention provides modified polymers ofintrinsic microporosity. For example, in some embodiments, a modifiedpolymer of intrinsic microporosity comprises a polymer of intrinsicmicroporosity that includes a plurality of repeat units, as describedabove.

Modifications to polymers of intrinsic microporosity may includereduction of one or more repeat units to generate a negative chargewithin the repeat unit. Thus, in some embodiments, at least one of therepeat units in a modified polymer of intrinsic microporosity includesone or more negative charges. Optionally, at least one of the repeatunits includes a negatively charged nitrogen site, a negatively chargedoxygen site, a negatively charged sulfur site, a negatively chargedcarbon site, or any combination thereof. The chemical identity andposition of such negatively charged sites may be determined by thespecific nucleophilic reaction that takes place to provide the negativecharge to the repeat unit.

FIG. 1B provides a schematic illustration of an embodiment of a modifiedpolymer of intrinsic microporosity 100B. Similar to FIG. 1A, themodified polymer of intrinsic microporosity 100B includes a plurality ofrepeat units 101B, 102B, and 103B linked by non-rotatable bonds andoptionally including a non-rotatable linker between different repeatunit portions. Here, the modified polymer of intrinsic microporosity100B is depicted as including a plurality of negative charges 108. Suchnegative charges may be formed by chemical reduction of portions of therepeat units. As illustrated, repeat unit 101B includes two negativecharges, repeat unit 102B includes two negative charges, and repeat unit103B includes one negative charge. It will be appreciated that eachrepeat unit may include more or fewer negative charges, as may beintroduced through the chemical reduction process. It will further beappreciated that positive counter ions may be present and or coordinatedwith the negative charge. For simplicity, any positive counter ions arenot illustrated.

In some embodiments, at least one repeat unit in a modified polymer ofintrinsic microporosity includes a charged moiety selected from thegroup consisting of:

Optionally, at least one of the repeat units in a modified polymer ofintrinsic microporosity has a structure selected from the groupconsisting of:

Modifications to polymers of intrinsic microporosity may alternativelyor additionally include reaction of one or more repeat units with anucleophile to generate a negative charge within the repeat unit.Accordingly, in some embodiments, at least one of the repeat units in amodified polymer of intrinsic microporosity includes one or morenegative charges. Optionally, at least one of the repeat units includesa negatively charged nitrogen site, a negatively charged oxygen site, anegatively charged sulfur site, a negatively charged carbon site, or anycombination thereof. The chemical identity and position of suchnegatively charged sites may be determined by the specific nucleophilicreaction that takes place to provide the negative charge to the repeatunit.

FIG. 1C provides a schematic illustration of an embodiment of a modifiedpolymer of intrinsic microporosity 100C. Similar to FIGS. 1A-1B, themodified polymer of intrinsic microporosity 100C includes a plurality ofrepeat units 101C, 102C, and 103C linked by non-rotatable bonds andoptionally including a non-rotatable linker between different repeatunit portions. Here, the modified polymer of intrinsic microporosity100C is depicted as including a plurality of negative charges 108. Suchnegative charges may be formed by chemical reaction of portions of therepeat units with a nucleophile, such as a nucleophilic additionreaction. In a nucleophilic addition reaction, a portion of anucleophile may be incorporated into the polymer of intrinsicmicroporosity, illustrated in FIG. 1C as charged moiety 109. Asillustrated, repeat unit 101C includes two negative charges, repeat unit102C includes one negative charge, and repeat unit 103C includes twonegative charges. It will be appreciated that each repeat unit mayinclude more or fewer negative charges, as may be introduced through thechemical nucleophilic reaction process. It will further be appreciatedthat positive counter ions may be present and or coordinated with thenegative charge. Again, for simplicity, any positive counter ions arenot illustrated.

In some embodiments, at least one repeat unit in a modified polymer ofintrinsic microporosity includes a charged moiety selected from thegroup consisting of:

where subscript m is an integer selected from 1 to 8, or

where subscript m and subscript o are independently integers selectedfrom 1 to 8. Optionally, at least one of the repeat units in a modifiedpolymer of intrinsic microporosity has a structure selected from thegroup consisting of:

where subscript m and subscript o are independently integers selectedfrom 1 to 8.

Modifications to polymers of intrinsic microporosity may alternativelyor additionally include crosslinking of the repeat units. For example,in some embodiments, at least one repeat unit is crosslinked with anon-adjacent repeat unit, such as a repeat unit of a different polymerchain. Crosslinking may be achieved by exposure of a polymer ofintrinsic microporosity to a crosslinking agent. Crosslinking agents mayinclude heating the polymer of intrinsic microporosity or exposing thepolymer of intrinsic microporosity to ultraviolet and/or microwaveradiation. Crosslinking agents may alternatively or additionally includecompounds that may react with multiple repeat units or may facilitatereaction between repeat units. It will be appreciated that, becauseadjacent repeat units are already covalently bonded to one another, suchas through a non-rotatable bond, crosslinking may refer to the creationof covalent links between non-adjacent repeat units.

FIG. 1D provides a schematic illustration of an embodiment of a modifiedpolymer of intrinsic microporosity 100D. In FIG. 1D, two polymersegments are illustrated, which may belong to the same polymer moleculeor different polymer molecules. A plurality of covalent bonds 110 areillustrated as bonding between non-adjacent repeat units. In FIG. 1D, acovalent crosslinking moiety 111 is illustrated as linking non-adjacentrepeat units. It will be appreciated that FIG. 1D is to be interpretedas not limiting the number and location of crosslinks in a modifiedpolymer of intrinsic microporosity, which may be between repeat unitportions, or between a non-rotatable linker and a repeat unit portion,or between non-rotatable linkers, for example.

Optionally, at least one repeat unit is crosslinked with a non-adjacentrepeat unit by a crosslinker selected from the group consisting of2,6-bis(4-azidobenzylidene)cyclohexanone,2,6-bis(4-azidobenzylidene)-4-methylcyclohexanone,2,6-bis(4-azidobenzylidene)-4-ethylcyclohexanone, 4-azidophenylsulfone,and any combination of these.

In some embodiments, methods of making modified polymers of intrinsicmicroporosity are provided. In one embodiment, a method of making amodified polymer of intrinsic microporosity comprises forming a reactionmixture comprising a polymer of intrinsic microporosity and a reducingagent, or a nucleophile, or a crosslinking agent, or any combinationthereof under conditions sufficient to form the modified polymer ofintrinsic microporosity. In some embodiments, the modified polymer ofintrinsic microporosity comprises a plurality of repeat units and atleast one of the repeat units includes one or more negative charges, orwherein the modified polymer of intrinsic microporosity is crosslinked,or both.

FIG. 2 provides an overview of some method embodiments for makingmodified polymers of intrinsic microporosity. A reaction mixturecomprising a polymer of intrinsic microporosity 200A is formed. Thereaction mixture may further include one or more of a reducing agent210, a nucleophile 220, or a crosslinking agent 230. Reaction betweenthe polymer of intrinsic microporosity 200A and the reducing agent 210or the nucleophile 220 may generate one or more negative charges in atleast one of the repeat units.

When a mixture of a polymer of intrinsic microporosity 200A and reducingagent 210 is formed, reaction 215 may occur to form modified polymer ofintrinsic microporosity 200B that includes negative charges incorporatedinto one or more of the repeat units. When a mixture of a polymer ofintrinsic microporosity 200A and nucleophile 220 is formed, reaction 225may occur to form modified polymer of intrinsic microporosity 200C thatincludes negative charges incorporated into one or more of the repeatunits.

Useful reducing agents include, but are not limited to an alkali metalpolysulfide, such as Li₂S_(m), where subscript m is an integer selectedfrom 2 to 100, an alkali metal sulfide, such as Li₂S, ammonium sulfide,an alkali metal hydrogen sulfide, such as LiSH, an alkali metal, ametallocene, such as (C₅Me₅)₂Fe or (C₅Me₅)₂Ni, an alkali metalnaphthalenide, such as NaC₁₀H₈ or LiC₁₀H₈, an inorganic reducing agenthaving an oxidation potential at or below 0.0 V vs. a standard hydrogenelectrode (SHE), an organic reducing agent having an oxidation potentialat or below 0.0 V vs. SHE, and any combination of these.

Useful nucleophiles include, but are not limited to, an alkali metalpolysulfide, Li₂S_(m) where subscript m is an integer selected from 2 to100, an alkali metal sulfide, ammonium sulfide, an alkali metal hydrogensulfide, an alkali metal alkylsulfide, an alkali metal arylsulfide,P₂S₅, an alkali metal sulfite, and any combination of these.

Any suitable solvent can be used in the methods of the presentinvention. Representative solvents include, but are not limited to,glyme or dimethoxymethane based solvents, such as diglyme (G2), triglyme(G3), and tetraglyme (G4), pentane, pentanes, hexane, hexanes, heptane,heptanes, petroleum ether, cyclopentanes, cyclohexanes, benzene,toluene, xylene, trifluoromethylbenzene, halobenzenes such aschlorobenzene, fluorobenzene, dichlorobenzene and difluorobenzene,methylene chloride, chloroform, or combinations thereof. In someembodiments, the solvent can be pentanes, hexanes, heptanes,cyclopentanes, cyclohexanes, benzene, toluene, xylene,trifluoromethylbenzene, chlorobenzene, or combinations thereof. In someembodiments, the solvent can be pentanes, hexanes, heptanes,cyclopentanes, cyclohexanes, or combinations thereof. In someembodiments, the solvent can be pentanes, hexanes, heptanes, orcombinations thereof. In some embodiments, the solvent can be heptanes.In some embodiments, the solvent can be toluene. In some embodiments,the reaction mixture can be a heterogeneous reaction mixture. In someembodiments, the reaction mixture can be a suspension.

The reaction mixture of the method can be at any suitable temperature.For example, the temperature of the reaction mixture can be of fromabout −78° C. to about 100° C., or of from about −50° C. to about 100°C., or of from about −25° C. to about 50° C., or of from about −10° C.to about 25° C., or of from about 0° C. to about 20° C. In someembodiments, the temperature of the reaction mixture can be of fromabout −25° C. to about 50° C. In some embodiments, the temperature ofthe reaction mixture can be of from about −10° C. to about 25° C. Insome embodiments, the temperature of the reaction mixture can be of fromabout 0° C. to about 20° C.

The reaction mixture of the method can be at any suitable pressure. Forexample, the reaction mixture can be at atmospheric pressure. Thereaction mixture can be also be exposed to any suitable environment,such as atmospheric gases, or inert gases such as nitrogen or argon.

When a mixture of a polymer of intrinsic microporosity 200A andcrosslinking agent 230 is formed, crosslinking reaction 235 may occur toform modified polymer of intrinsic microporosity 200D that includes oneor more crosslinks between non-adjacent repeat units. For example, in anembodiment, reaction between the polymer of intrinsic microporosity andthe crosslinking agent induces crosslinking of the polymer of intrinsicmicroporosity by generating one or more covalent bonds between a firstrepeat unit and a second repeat unit that is not adjacent to the firstrepeat unit

Useful crosslinking agents include, but are not limited to,2,6-bis(4-azidobenzylidene)cyclohexanone,2,6-bis(4-azidobenzylidene)-4-methylcyclohexanone,2,6-bis(4-azidobenzylidene)-4-ethylcyclohexanone, 4-azidophenylsulfone,oxygen, and any combination of these.

Optionally, forming a reaction mixture comprising a polymer of intrinsicmicroporosity and a crosslinking agent comprises inducing a crosslinkingreaction of the polymer of intrinsic microporosity by exposure of thepolymer of intrinsic microporosity to one or more of ultravioletradiation, microwave radiation, and heat.

Optionally, polymers of intrinsic microporosity and modified polymers ofintrinsic microporosity may be formed into membranes. For example, thepolymers may be cast into a membrane comprising a web or sheet of thepolymer material. Optionally, a membrane comprising a polymer ofintrinsic microporosity or a modified polymer of intrinsic microporositymay comprise a support membrane. In some embodiments, the polymer ofintrinsic microporosity or a modified polymer of intrinsic microporositymay be cast to, coated on, and/or encapsulate the support membrane. Useof a support membrane may be beneficial, in some embodiments, when thepolymer of intrinsic microporosity or the modified polymer of intrinsicmicroporosity does not possess sufficient strength to form afree-standing film or if additional strength or features are desired ina membrane. For example, use of a polymeric support membrane may beuseful for imparting a membrane with mechanical strength or to provideparticular thermal characteristics to the membrane. Such properties maybe useful for electrochemical cells and ion separation systemscomprising a polymer of intrinsic microporosity or a modified polymer ofintrinsic microporosity.

In some embodiments, the present invention provides modified polymers ofintrinsic microporosity prepared by reacting a polymer of intrinsicmicroporosity with a reducing agent, a nucleophile, a crosslinkingagent, or any combination of these. Optionally, reacting a polymer ofintrinsic microporosity with a reducing agent, a nucleophile, and/or acrosslinking agent includes forming a reaction mixture of the polymer ofintrinsic microporosity and the reducing agent, the nucleophile, and/orthe crosslinking agent under conditions sufficient to form the modifiedpolymer of intrinsic microporosity. Sufficient conditions may include,but are not limited to, a temperature adequate for the reaction toproceed, such as a temperature of about 25° C., a pressure adequate forthe reaction to proceed, such as a pressure of about atmosphericpressure, and adequate reactant ratios, such as a molar ratio of thepolymer of intrinsic microporosity to the reducing agent, thenucleophile, and/or the crosslinking agent selected from the range ofabout 100:1 to about 1:100, for example.

In some embodiments, the present invention provides films comprisingmodified polymers of intrinsic microporosity prepared by forming a filmof a polymer of intrinsic microporosity and exposing the film to areducing agent, a nucleophile, and/or a crosslinking agent. It will beappreciated that films may be formed using any techniques, such as adrop casting technique or a dip coating technique. In an embodiment, afilm comprising a modified polymer of intrinsic microporosity isprepared by generating a mixture by dissolving the polymer of instrinsicmicroporosity in a solvent, placing the mixture onto a surface,evaporating the solvent present in the mixture to form the film, andexposing the film to film to a reducing agent, a nucleophile, and/or acrosslinking agent. Optionally, the surface may be a polymer supportstructure, such as a porous polymer support.

Optionally, a modified polymer of intrinsic microporosity is prepared byreacting a polymer of intrinsic microporosity with a nucleophile and/ora reducing agent and then crosslinking the modified polymer of intrinsicmicroporosity. Optionally, a modified polymer of intrinsic microporosityis prepared by crosslinking the polymer of intrinsic microporosity andthen reacting the crosslinked polymer of intrinsic microporosity with anucleophile and/or a reducing agent. Crosslinking methods useful forpreparing modified polymers of intrinsic microporosity include exposinga polymer of intrinsic microporosity or a modified polymer of intrinsicmicroporosity to ultraviolet, exposing a polymer of intrinsicmicroporosity or a modified polymer of intrinsic microporosity tomicrowave radiation, and/or heating a polymer of intrinsic microporosityor a modified polymer of intrinsic microporosity.

IV. ELECTROCHEMICAL CELLS

In some embodiments, electrochemical cells are provided herein.Electrochemical cells of some embodiments may comprise a polymer ofintrinsic microporosity or a modified polymer of intrinsicmicroporosity, such as described above. In a specific embodiment, forexample, an electrochemical cell comprises an anode, an anodeelectrolyte in contact with the anode, a separator in contact with theanode electrolyte, such as a separator that comprises a polymer ofintrinsic microporosity, a cathode electrolyte in contact with theseparator, and a cathode in contact with the cathode electrolyte.Optionally, the polymer of intrinsic microporosity comprises a modifiedpolymer of intrinsic microporosity, such as described above.

FIG. 3 provides a schematic illustration of an electrochemical cell 300embodiment. Electrochemical cell 300 comprises an anode 305, an anodeelectrolyte 310, a separator 315, a cathode electrolyte 320 and acathode 325. As a specific example, electrochemical cell 300 maycomprise a lithium-sulfur battery, where anode 305 comprises lithium andcathode 325 comprises sulfur.

It will be appreciated that, while anode 305 is illustrated as anexplicit body of material, in some embodiments, anode 305 may beincluded within anode electrolyte 310, such as a dissolved componentwithin anode electrolyte 310, as may be the configuration in a flowbattery. It will also be further appreciated that, while cathode 325 isillustrated as an explicit body of material, in some embodiments, anode325 may be included within cathode electrolyte 320, such as a dissolvedcomponent within cathode electrolyte 320, as may be the configuration ina flow battery.

Optionally, the separator 315 comprises a support membrane 318 incontact with the polymer of intrinsic microporosity, such as describedabove. Useful support membranes include those comprising a polymerselected from the group consisting of: polyethylene, polyethylenecopolymers, polypropylene, polypropylene copolymers, polyacrylonitrile,polyacrylonitrile copolymers, poly(vinylidene fluoride),poly(tetrafluoroethylene), poly(vinyl chloride), poly(vinylchloride)copolymers, poly(hexafluoropropylene), poly(hexafluoropropylene)copolymers, polyaramide, any combination thereof, and any copolymersthereof.

As illustrated in FIG. 3, separator 315 includes two support membrane318. While two support membranes are illustrated in FIG. 3, it will beappreciated that any number of support membranes are useful withelectrochemical cells of some embodiments. Alternatively, someseparators may not include any support membrane. Support membrane 318may comprise a porous polymer film, which may allow ions to pass throughunimpeded. Optionally, electrochemical cell may include one or morecurrent collectors, such as a first current collector in contact withanode 305 and/or anode electrolyte 310, and/or a second currentcollector in contact with cathode electrolyte 320 and/or cathode 325.

In some embodiments, the support membrane 318 has a melting temperature,and exposing the support membrane 318 to a temperature exceeding themelting temperature causes at least a portion of the support membrane318 to melt and close pores within the separator. For example, themelting temperature may be selected from the range of 100° C. to 180° C.

Use of a support membrane that may advantageously provide importantsafety features to electrochemical cells incorporating such. Forexample, a support membrane may provide additional strength to theseparator beyond that provided by the polymer of intrinsic microporosityor modified polymer of intrinsic microporosity comprising the separator.Such strength may be important for blocking dendrites that may form onthe anode, for example, from puncturing the separator and contacting thecathode. Such a condition may result in overheating or thermal runawayof an electrochemical cell and potentially ignite the electrolyte orenclosure components.

Additionally or alternatively, a support membrane that may melt duringheating caused by damage to a separator, thermal runaway, and/oroverheating may advantageously stop the thermal runaway and/oroverheating. For example, melting of at least a portion of the supportmembrane may result in pores in the separator closing, which may stopany discharge of the electrochemical cell. Additionally, melting of atleast a portion of the support membrane may coat a dendrite that hasdamaged the separator, preventing the dendrite from making contact withthe cathode, for example.

V. SELECTIVE ION TRANSPORT TECHNIQUES AND ION SEPARATION SYSTEMS

In some embodiments, ion separation systems and methods of selective iontransport are provided herein. Ion separation systems of someembodiments may comprise a polymer of intrinsic microporosity or amodified polymer of intrinsic microporosity, such as described above. Anexample ion separation system 400 is depicted in FIG. 4A. Ion separationsystem 400 comprises a first ionic solution 410 comprising a first ionicspecies (I1′), a separator 415 in contact with the first ionic solution,such as a separator comprising a polymer of intrinsic microporosityand/or a modified polymer of intrinsic microporosity, and a second ionicsolution 420 in contact with the separator, such as a second ionicsolution that includes a second ionic species (I2′). As illustrated afirst side 416 of the separator 415 is in contact with first ionicsolution 410 and a second side 417 of the separator 415 is in contactwith second ionic solution 420. The separator 415 may be selectivelypermeable to the first ionic species such that the first ionic speciesmay transport between the first ionic solution 410 and the second ionicsolution 420 through micropores of the separator. The separator 415 mayrestrict the second ionic species from passing through the separator 415to reach the first ionic solution. For example, the micropores of theseparator 415 may have a cross sectional dimension that provides sizesieving and restriction of the second ionic species from beingtransported through the separator. In separators including a modifiedpolymer of intrinsic microporosity that include negatively chargedsites, the negative charges may further provide an electrostaticrestriction on the second ionic species from being transported throughthe separator.

Systems and methods of some embodiments may be useful for restrictingions of a particular size and/or a particular size and chargecombination to a particular region. For example, it may be desirable tolimit the passage of polysulfide ions from a cathode electrolyte in anelectrochemical cell to the anode electrolyte in the electrochemicalcell. The systems and methods of some embodiments may advantageouslyfacilitate this limitation, such as by preventing the polysulfide ionsfrom passing through a separator and/or by reducing a rate at whichpolysulfide ions pass through a separator.

In a specific embodiment, a method of selective ion transport comprisescontacting a first side of a separator with a first ionic solution, suchas a separator that comprises a polymer of intrinsic microporosity,wherein the first ionic solution comprises a first ionic species;contacting a second side of the separator with a second ionic solution,wherein the second ionic solution comprises a second ionic species; andtransporting the first ionic species between the first ionic solutionand the second ionic solution through the separator. In someembodiments, the separator provides a size selective restriction ontransport of the second ionic species from the second ionic solution tothe first ionic solution through the separator. Optionally, theseparator further provides an electrostatic restriction on transport ofthe second ionic species from the second ionic solution to the firstionic solution through the separator.

VI. EXAMPLES

The invention may be further understood by reference to the followingnon-limiting examples.

Example 1 Polysulfide-Blocking Microporous Polymer Membrane Tailored forHybrid Li-Sulfur Flow Batteries

Redox flow batteries (RFBs) present unique opportunities for multi-hourelectrochemical energy storage (EES) at low cost. Too often, the barrierfor implementing them in large-scale EES is the unfettered migration ofredox active species across the membrane, which shortens battery lifeand reduces Coulombic efficiency. To advance RFBs for reliable EES, anew paradigm for controlling membrane transport selectivity is needed.This Example shows that size- and ion-selective transport can beachieved using membranes fabricated from polymers of intrinsicmicroporosity (PIMs). For example, a first-generation PIM membranedramatically reduced polysulfide crossover (and shuttling at the anode)in lithium-sulfur batteries, even when sulfur cathodes were prepared asflowable energy-dense fluids. The design of the membrane platform wasinformed by molecular dynamics simulations of the solvated structures oflithium bis(trifluoromethanesulfonyl)imide (LiTF SI) vs lithiatedpolysulfides (Li₂S_(x), where x=8, 6, and 4) in glyme-based electrolytesof different oligomer length. These simulations suggested polymer filmswith pore dimensions less than 1.2-1.7 nm might incur the desiredion-selectivity. Indeed, the polysulfide blocking ability of the PIM-1membrane (˜0.8 nm pores) was improved 500-fold over mesoporous Celgardseparators (˜17 nm pores). As a result, significantly improved batteryperformance was demonstrated, even in the absence of LiNO₃anode-protecting additives.

Membranes (or separators) are critical for ionic conduction andelectronic isolation in many electrochemical devices. For cellarchitectures that utilize redox-active species that are dissolved,dispersed, or suspended in electrolyte, from fuel cells (FCs) to redoxflow batteries (RFBs), it is also of value that the membrane preventactive material crossover that would otherwise contribute to deviceshorting, electrode fouling, or irrevocable loss in capacity.Unfortunately, commercial battery separators, which featureshape-persistent mesopores, are freely permeable to most activematerials used in RFBs. Alternative membrane separators have thus farrelied heavily on variants of aqueous single-ion conductors, e.g.,Nafion, which may ultimately restrict the use of certain types offlowable electrodes. Considerably less attention has been paid tosize-sieving as a mechanism to achieve membrane selectivity, althoughsuccess in this regard would allow greater flexibility in batterychemistries. Despite the wide availability of porous materials thatmight serve effectively as membrane components, including zeolites,metal-organic frameworks, covalent organic frameworks, carbon nanotubes,cyclic peptide nanotubes, and microporous polymers, rational designrules for achieving ion-selective transport via sieving in flow batterymembranes have not been established.

Guided by theoretical calculations, this example applies polymers ofintrinsic microporosity (PIMs) as a membrane platform for achievinghigh-flux, ion-selective transport in nonaqueous electrolytes. Thesepolymers are synthesized in a single step and easily cast intolarge-area sheets with well-controlled pore structure and pore chemistry(FIG. 5). The unique micropore architecture of PIMs arises primarilyfrom two molecular characteristics: (1) PIMs do not feature rotatingbonds along their backbone; and (2) they incorporate rigid sharp bendsinto at least one of the constituent monomers at regular intervals alongthe polymer chain. Both features contribute to frustrated packing ofpolymer chains in the solid state. As a result, PIMs are amorphous yetexhibit high intrinsic microporosity (<2 nm) and high surface area(300-1500 m² g⁻¹). The open pore architecture of PIMs suggests that theymight be advantageously positioned for selective species transport inelectrochemical devices via sieving.

This Example highlights new opportunities for PIMs to serve asion-selective membranes in RFBs, using lithium-sulfur (Li—S) as a modelbattery chemistry. Here the lithium anode is stationary and separated,by the membrane, from the flowable sulfur-containing catholyte. This RFBfeatures a high theoretical specific energy capacity of 1670 mAh g⁻¹ forS and operating voltage that exceeds 2.0 V. While these are desirablecharacteristics, this battery chemistry suffers from low Coulombicefficiency and rapid capacity fade when lithium polysulfides (PS)crossover to and react with the metal anode surface. Strategies seekingto mitigate PS crossover in Li—S batteries have included the use ofsacrificial anode-protecting additives (e.g., LiNO₃), single-ionconducting membranes, conductive interlayers, permselective barriers,and even polysulfide adsorbates. Nonetheless, continuous Li consumptionupon cycling remains a problem. This Example demonstrates that PIMmembranes block PS crossover while allowing ions in the supportingelectrolyte to traverse the membrane with minimal impedance andindicates a direct solution to the PS crossover problem is feasible.This Example also shows dramatically improved performance of batterieswhen PIM membranes are in place, rather than conventional batteryseparators.

To inform the rational design of a membrane platform capable ofachieving high transport selectivity for supporting electrolyte(lithiumbis(trifluoromethane)sulfonimide, LiTFSI) vs PS in Li—S RFBs,molecular dynamics (MD) simulations were carried out for each species'solvated structures in different ethereal solvents, diglyme (G2),triglyme (G3), and tetraglyme (G4), as these are commonly used in Li—SRFBs. The simulated effective sizes of these solvated complexes weredetermined by the radii of gyration (R_(g)) of the solute and the firstsolvation shell. These shells were typically composed of two solventmolecules, as exemplified by the average snapshots shown in FIG. 6A. Thesize of elemental sulfur was calculated, which exhibits no explicitsolvent coordination in the simulations. For this singular case, a sizefor S₈ was determined using its atoms' van der Waals solvent-excludedradii. The determinations of R_(g) provide size-ranges for selective iontransport (FIG. 6B). As the primary contributors to the shuttlingcurrents are lithium polysulfides, Li₂S_(x) where x≥4, the membrane poredimensions should be smaller than 1.2-1.7 nm in order to achieveion-selective transport.

Directed by the MD simulations, PIM-130 was identified as a possiblePS-blocking membrane material for Li—S hybrid flow cells. PIM-1 is theprogenitor of a family of non-networked ladder polymers that aremechanically and thermally robust; pertinent to their use here, theirpore dimensions are sub-nm. PIM-1 was synthesized (200 kg mol⁻¹) on amultigram scale from inexpensive, commercially available monomers andcast from solution into flexible free-standing membranes (˜10 μm thick)(FIG. 5 and FIG. 9). The specific surface area (795 m² g⁻¹) and poresize distribution of PIM-1 was determined using nitrogen adsorptionisotherms (FIG. 6C). PIM-1 membranes had a nominal pore size of 0.77 nm,which is useful for selective transport of LiTFSI and PS blocking. Thisstands in stark contrast with commercially available Celgard 2325, whichhas a much larger pore size of 17 nm: far too large for size-selectivetransport (FIG. 6C). Celgard 2325 and similar mesoporous polymerseparators are commonly used in Li—S cells and serve as a usefulbenchmark for new membrane materials. A total porosity of ˜25% wasdetermined for PIM-1 membranes using ellipsometric porosimetry, which iscomparable to the porosity of Celgard 2325. As PIM-1 membranes areexpected to swell to a degree upon introduction of electrolyte, thisdetermination should be considered a lower limit to the available freevolume.

It was hypothesized that during battery operation the free volume inPIM-1 (and PIMs generally) would become swollen and infiltrated withelectrolyte, creating an ionically percolating solution-phase conductivenetwork. As a result, ion flux would be solely carried by (and bedependent on) the solution conductivity within the pores; polymer chaindynamics, which are orders of magnitude slower, would no longer dictatethe membrane's ionic conductivity. To test this hypothesis, PIM-1'smembrane ionic conductivities were evaluated in glymes of differentoligomer lengths, diglyme (G2), triglyme (G3), and tetraglyme (G4),containing 0.50 M LiTFSI. A strong correlation between the membraneionic conductivity and the bulk solution ionic conductivity of theelectrolyte was noted (FIG. 7A). These results indicate that the ioncurrent is indeed carried by the infiltrating electrolyte, as predicted.This behavior was also observed in Celgard separators (FIG. 7A). Bycomparing the membrane ionic conductivities for Celgard and PIM-1, itwas found that reducing the pore dimensions from 17 to 0.77 nm,respectively, only decreased membrane ionic conductivity 10-fold. It wasalso found that electrolytes based on diglyme provided the highestmembrane ionic conductivity for both platforms and was thus chosen asthe supporting electrolyte for all subsequent experiments.

To quantify the polysulfide-blocking ability of PIM-1 vs Celgard,membrane crossover experiments were performed in H-cells configured withdissolved PS (2.5 M S as Li₂S₈ in diglyme containing 0.50 M LiTFSI and0.15 M Li_(N)O₃) on the retentate side and PS-free electrolyte on thepermeate side (FIG. 7B, inset). The concentration of PS over time wasthen monitored electrochemically on the permeate side using eithercyclic voltammetry or square wave voltammetry, where current could becorrelated to concentration of PS via a calibration curve (FIG. 10 andFIG. 11). Using an initial rate approximation, the diffusion coefficientof PS across the membranes were calculated to be 6.8×10⁻⁸ cm² S⁻¹ forCelgard and 1.3×10⁻¹⁰ cm² s⁻¹ for PIM-1 (˜500-fold reduction). This iscompelling evidence that PS are screened by a size-sieving mechanismwithin PIM-1's ionically percolating micropore network, as hypothesized.This PS-blocking ability comes at minimal expense to overall membraneionic conductivity compared to Celgard, thus highlighting the value inguiding membrane design through careful examination of the solvatedstructures of ions vs redox active species in the electrolyte.

Given the outstanding PS-blocking ability of the PIM-1 membrane, theirperformance in Li—S batteries was tested employing soluble sulfurcatholytes. To do so, Swagelok cells were assembled with Li-metalanodes, polysulfide catholytes (2.5 M S as Li₂S₈ in diglyme containing0.50 M LiTFSI), and Celgard or PIM-1 membranes. Lithium anodes werescraped to reveal a fresh surface prior to cell assembly. Seeking toisolate the membrane's influence on mitigating PS shuttling currents,LiNO₃ additives were deliberately avoided in the electrolyteformulation. Moreover, to improve sulfur utilization, 5 wt % Ketjenblackwas employed as an embedded current collector in the catholyte. Threebreak-in cycles at C/10 were used to equilibrate PIM-1's membranemicroenvironments before cycling at a C/8 rate. Overall, higher capacityfade was observed for both types of cells during the break-in due to theample time allowed for polysulfide shuttling. The Li—S cells configuredwith Celgard membranes exhibited a drastic capacity fade from ˜150 WhL⁻¹ after the break-in cycles to less than 20 Wh L⁻¹ within the first 20cycles, all at a C/8 rate. In contrast, batteries configured with PIM-1membranes exhibited higher capacity at all cycles, sustaining 50 Wh L⁻¹at the end of 50 cycles (FIG. 8A). The performance of PIM-1 membraneswas further improved with the addition of LiNO₃ as an anode-protectingadditive, with a sustained capacity of approximately 100 Wh L⁻¹ after 50cycles (FIG. 8A) and stable cycling at rates as high as C/4 (FIG. 8B).These results represent improvements in capacity retention over relatedwork with Li—S flow cells, particularly in the absence of LiNO₃, andhighlight the possibility for combining the disclosed membrane approachwith other strategies to mitigate the effects of polysulfide crossover.

Redox flow batteries present unique opportunities for low-cost,multi-hour energy storage. In order for RFBs to mature as a deployabletechnology, their longevity should be greatly improved for batterychemistries offering high-power performance. Toward that end, thisExample highlighted the transport needs for membranes employed innonaqueous Li—S cells, where the cathode was formulated as anenergy-dense, flowable solution of polysulfides with Ketjenblack as anembedded current collector. It was showed that rational principles formembrane design emerge from molecular dynamics simulations of thesolvated structures of S₈, Li₂S_(x) (x=8, 6, or 4), and LiTFSI indifferent electrolytes, and more specifically, that their calculatedradius of gyration places an upper limit of 1.2-1.7 nm on the poredimensions required for polysulfide blocking. Indeed, this Exampleshowed that membranes processed from polymers of intrinsic microporosityexhibited unprecedented blocking characteristics for solublepolysulfides owing to their sub-nm pore dimensions. This blockingability led to significantly improved device performance with respect tocapacity fade and other important metrics. Given that the pore size,pore chemistry, and overall porosity for PIM membranes are tunable usingmolecular engineering and polymer processing, the membrane's transportcharacteristics can be tailored to suit a broad spectrum ofelectrochemical devices, including stationary batteries and fuel cells.

Figure Captions. FIG. 5: Ion-selective transport across membranesfabricated from PIM-1. For Li—S batteries, both stationary and hybridflow, blocking Li₂S_(x) (where x≥4) crossover is critical to sustainingpeak battery performance. Membranes based on PIM-1 achieve hightransport selectivity for LiTFSI by reducing the membrane poredimensions to subnanometer regimes, which shuts down polysulfidecrossover via a sieving mechanism. Ion flux across the membrane is tiedto overall microporosity, pore architecture, and electrolyteformulation.

FIG. 6A: Snapshots from MD simulations nearest to the average size ofsolvated LiTFSI and Li₂S_(x) (x=4, 6, and 8) in diglyme, triglyme, andtetraglyme. FIG. 6B: Calculated radii of gyration (R_(g)) for Li₂S₄,Li₂S₆, and Li₂S₈, along with their first solvation shells, in diglyme,triglyme, and tetraglyme as determined by MD simulations. FIG. 6C: Poresize distributions for microporous PIM-1 vs mesoporous Celgard polymermembranes.

FIG. 7A: Ambient temperature ionic conductivity of microporous PIM-1 vsmesoporous Celgard membranes infiltrated with different electrolyteformulations: 0.50 M LiTFSI in diglyme (G2), triglyme (G3), ortetraglyme (G4). FIG. 7B: Time-evolution of the concentration of PS inthe permeate (left) of H-cells configured with either a Celgard (black)or a PIM-1 (green) membrane. The retentate was charged with an initialconcentration of 2.5 M S as Li₂S₈ in diglyme containing 0.50 M LiTFSIand 0.15 M LiNO₃. The concentration of PS in the permeate was determinedelectrochemically.

FIG. 8A: Volumetric energy density as a function of cycle number forCelgard membrane with no LiNO₃ (black circles), PIM-1 membrane with noLiNO₃ (light green circles), and PIM-1 membrane with LiNO₃ additive(dark green circles). FIG. 8B: Rate performance of PIM-1 membrane withLiNO₃ additive.

Experimental Details. Materials: Tetraglyme (G4), triglyme (G3), diglyme(G2), 3,3,3′,3′-tetramethyl-1,1′-spirobiindane-5,5′,6,6′-tetraol andtetrafluoroterephthalonitrile were purchased from Sigma Aldrich. Lithiumnitrate, sulfur (Puratronic, 99.9995% (metals basis)), lithium sulfide(99.9% (metals basis)), and lithium metal (99.9% (metals basis), 1.5 mm)were purchased from Alfa Aesar. Lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI) was purchased from 3M.Celgard 2535 membrane was purchased from MTI Corporation. KetjenblackEC-600JD was purchased from AkzoNobel.

General methods: Nuclear magnetic resonance (NMR) spectra were taken ona Bruker Avance II 500 MHz NMR spectrometer. Analysis of the polymer'smolecular weight distribution was carried out using size exclusionchromatography on a Malvern Viscotek TDA 302 system. Residual watercontent for various solvents was determined by a Mettler Toledo C20Coulometric Karl Fischer titrator. Electrochemical experiments andbattery testing were conducted with a BioLogic VMP3 potentiostat.Scanning electron micrographs were obtained with a Zeiss Gemini Ultra-55analytical scanning electron microscope equipped with in-lens andsecondary electron detectors at a beam energy of 2 keV. Ellipsometricporosimetry (EP) was performed on a Semilab PS-1100 instrument withtoluene or isopropanol.

Electrode details: Swagelok batteries were constructed using Swagelokunions purchased from Swagelok Northern California. Associatedelectrodes were made in-house from nickel 200 rods with outer diametersof 1.27 cm. Wells, which were 0.635 cm in diameter and 0.508 mm deep,were machined into the cathode current collectors. Gold was sputteredonto the cathode current collector surface. Anode current collectorswere flat, bare nickel 200 surfaces.

Membrane preparation: PIM-1 was synthesized using a literatureprocedure. PIM-1 was dissolved in chloroform at a concentration of 12.5mg mL⁻¹. Films of PIM-1 were cast by depositing 1 mL of solution into a3.5 cm diameter Teflon well. The solvent was left to evaporate in aclosed vacuum chamber under ambient pressure for 1 h or until dryness.The films were further dried in vacuo overnight. The dried films werepunched into 7/16-inch circles. Celgard 2325 membranes were punched into½-inch circles. All membranes were soaked in relevant electrolytesovernight before use.

Ionic conductivity measurements: Soaked membranes were sandwichedbetween two stainless steel blocking electrodes. Potentioelectrochemical impedance spectroscopy (PEIS) was used with 50 mV ACbias scanning from 1 MHz to 100 mHz. The high frequency x-axis interceptis taken to be the resistance of the membrane. The membrane conductivitywas then calculated taking into account the cell geometry.

Electrolyte and polysulfide preparation: The supporting electrolyteformulation for all battery cycling and conductivity measurements was0.50 M LiTFSI. LiNO₃ was added to the electrolyte only for the crossoverexperiments detailed below. LiTFSI was dried for 16 h under vacuum at150° C. LiNO₃ was dried for 16 h under vacuum at 110° C. Diglyme wastested for peroxides prior to use; if any were measured, it was stirredwith alumina, filtered, and sparged with argon. Diglyme was dried withactivated 3 Å molecular sieves until it measured <20 ppm H₂O.Electrolyte was tested for water content and confirmed to contain <30ppm water before use. Solutions of Li₂S₈ (2.50 mol S L⁻¹ in electrolyte)were prepared by mixing Li₂S (0.287 g, 6.25 mmol), sulfur (1.40 g, 5.47mmol), and 20 mL of electrolyte and heating at 60° C. until all solidswere dissolved. Li₂S₈ solutions were kept at 60° C. in order to preventprecipitation of insoluble species and cooled to room temperature priorto use. Cathode slurry with 5% w/w conductive additive was made byadding 30.8 mg of Ketjenblack to 500 μL of Li₂S₈ solution and mixed for15 min.

Crossover experimental methods: Crossover measurements were made byplacing respective membranes between the cell halves of a PermeGearSide-Bi-Side diffusion cell. To the permeate side of the cell was added2.5 mL of supporting electrolyte (0.15 M LiNO₃, 0.5 M LiTFSI in diglyme)while to the retentate side was added 2.5 mL of 2.5 M S as Li₂S₈ inelectrolyte. In this case, due to the presence of lithium as a referenceelectrode, LiNO₃ was necessary to prevent the reaction of polysulfideswith the lithium. Crossover was determined by cyclic voltammetry andsquare wave voltammetry measurements of the permeate side of the cell.Cyclic voltammetry allowed concentrations between 5.0-60 mM to bemeasured while square wave voltammetry allowed for measurements ofconcentrations ranging from 0.20-1.0 mM. Given the different rates ofcrossover between the two materials, both techniques were necessary asthe Celgard crossover was too fast to be measured accurately with theSWV, and the PIM crossover was too slow to be measured in a convenienttime frame with CV. A glassy carbon disc electrode (1 mm) was obtainedfrom BAS Inc. (West Lafayette, Ind.), polished before use and used asthe working electrode. Lithium metal was used as the reference andcounter electrodes. A calibration curve for each electrochemicaltechnique was obtained by measuring the current as a function of voltagefor a set of known concentration polysulfide solutions (FIG. 9 and FIG.10). The concentration of polysulfide vs. time for the crossovermeasurements was then calculated using the linear equation determinedfrom the calibration curves.

Battery cycling: Cathode slurry was spread evenly into the cathode well.Lithium chip was punched using a 7/16-inch bore and pressed onto theanode. Due to the safety concern of dendrite formation, membranes weresandwiched between two Celgard layers to isolate them from the lithiumpolysulfide slurry and the lithium anode surface. The trilayer membranewas then pressed in between the two electrodes to assemble a Swagelokbattery.

Computational Methods. First-Principles molecular dynamics simulations:The S₈/Li-TFSI/Li₂S_(x)-TEGDME systems were simulated using a modifiedversion of the mixed Gaussian and plane wave code CP2K/Quickstep. Atriple-ζ basis set was employed with two additional sets of polarizationfunctions (TZV2P) and a 320 Ry plane-wave cutoff. The unknownexchange-correlation potential is substituted by the revised PBEgeneralized gradient approximation, and the Brillouin zone is sampled atthe Γ-point only. Interactions between the valence electrons and theionic cores are described by norm-conserving pseudopotentials. ThePoisson problem is tackled using an efficient Wavelet-based solver. Thepoor description of the short-range dispersive forces within the PBE-GGAexchange-correlation functional was overcome by employing DFTD3empirical corrections. In order to equilibrate the systems, 10 ps of NPTdynamics was performed, using a Nose-Hoover thermostat (temperaturedamping constant of 100 fs) and an Anderson barostat (pressure dampingconstant of 2 ps). Snapshots of the system were saved every step. Thesnapshot with a volume closest to the average of the last 5 ps of MD wasthen selected as input for an additional 20 ps simulation in theconstant volume, constant temperature (canonical or NVT) ensemble.

Structural analysis: The “size” of the solvated lithium polysulfidespecies was estimated as the sum of two terms: 1) the radius of gyrationof the solute (R_(gyr)) and 2) the size of the glyme solvation shell.All structural analyses were performed for every 10 snapshots from thelast 20 ps of the NVT AIMD simulations (4,000 for each system). TheR_(gyr) was computed as

$R_{gyr} = \sqrt{\frac{1}{M}{\sum\limits_{i}^{\;}{m_{i}( {r_{i} - r_{c\; m}} )}^{2}}}$

where M is the total mass of the solute, R_(cm) is the center of massand the sum is over all r_(i) atoms in the solute.

The solvation environment around each dissolved polysulfide was obtainedcalculating the Li-glyme (oxygen atom) and S-glyme pair distributionfunctions (PDF) from the last 20 ps NVT MD simulation. The 1st solvationshell was obtained from the minimum in the PDF after the first peak, andthe number of solvent molecules obtained by simple integration.

FIG. 9: Cross-sectional scanning electron micrograph of a freestandingPIM-1 membrane. The scale bar is 10 μm.

FIG. 10: Calibration curve of current vs. concentration obtained viasquare wave voltammetry for the lower concentration regime.

FIG. 11: Calibration curve of current vs. concentration obtained viacyclic voltammetry for the higher concentration regime.

FIG. 12: Electrochemical impedance spectroscopy (EIS) of Li—S cellsconfigured with PIM-1 and Celgard as membranes, respectively. Themembrane ionic conduction kinetics are represented by the sizes ofhigh-frequency semicircles, which are 20.1 Ohms and 215.1 Ohms forCelgard and PIM-1, respectively.

FIGS. 13A-13B: Volumetric energy densities of all batteries tested(catholyte formulation: 2.5 M S as Li₂S₈ in diglyme containing 0.50 MLiTFSI) with either PIM-1 membrane (green circles, FIG. 13A) or Celgardmembrane (purple circles, FIG. 13B).

FIGS. 14A-14B: Coulombic efficiencies of all batteries tested (catholyteformulation: 2.5 M S as Li₂S₈ in diglyme containing 0.50 M LiTFSI) witheither PIM-1 membrane (green circles, FIG. 14A) or Celgard membrane(purple circles, FIG. 14B).

FIGS. 15A-15C. Discharge and charge profiles for Li—S batteriesconfigured with: (FIG. 15A) PIM-1 membrane separators and LiNO₃electrolyte additive; (FIG. 15B) PIM-1 membrane separators without LiNO₃electrolyte additive; and (FIG. 15C) Celgard separators without LiNO₃additive at the 1st, 10th, 20th, 30th, 40th, and 50th cycles. The arrowsindicate the direction of higher cycle number.

FIG. 16: Representative Coulombic efficiency of a Li—S batteryconfigured with a PIM-1 membrane separator and LiNO₃ as an electrolyteadditive.

Additional description may be found in the article Nano Lett., 2015, 15(9), pp 5724-5729 (DOI: 10.1021/acs.nanolett.5b02078), and itsSupporting Information, which are hereby incorporated by reference.

Example 2 Ion- and Size-Selective Membranes for Electrochemical EnergyStorage Devices Based on Polymers with High Intrinsic Microporosity

Electrochemical energy storage (EES) devices rely on chemically-robustand ion-selective membranes to ensure durability. Despite the importanceof preventing membrane degradation for long-term battery operation,little is known about the design rules that tie local chemical changesin the membrane to evolution in its macroscopic structure andion-selectivity. This Example presents a strategy for discovering thesedesign rules, using the polysulfide-blocking ability of PIM-1 membranesas a model system of study. PIM-1 features electrophilic1,4-dicyanooxanthrene functionalities that are potentially subject tonucleophilic attack by lithium polysulfides, which are endogenous tolithium-sulfur batteries. This chemical reactivity was verified with insitu FT-IR experiments on the membrane as well as NMR and ESI-MSexperiments on model compounds representing monomer segments. It wasfound that it was advantageous to significantly increase the prevalenceof these polysulfide adducts, formally lithiated thioamides, in order toobserve a significant decrease in the membrane's polysulfide-blockingability, which were found to correlate to an increase in membrane poresize. This work suggests PIM-1 membranes perform optimally inlithium-sulfur batteries configured with composite sulfur cathodes,where the polysulfide concentration in electrolyte remains low.

Electrochemical energy storage (EES) devices rely on separators ormembranes to electrically isolate the negative and positive electrodeswhile allowing ionic current to flow between them. For EES devices withsolid-state electrodes (e.g., Li-ion or lithium-sulfur batteries),mesoporous polymer separators often serve this purpose. On the otherhand, for EES devices that use soluble active-materials (e.g., redoxflow batteries or lithium-polysulfide batteries), more advancedmembranes capable of blocking active-species crossover while allowingcounter-ions to pass may be useful. To this end, a number of membranematerials for selective lithium-ion transport in non-aqueouselectrolytes have been proposed, including lithiated Nafion, solidpolymer electrolytes, Li-ion conducting glasses, and polymers ofintrinsic microporosity (PIMs). These membranes must maintain theiractive-species blocking ability to ensure long lifetimes and highefficiency, even if those active-species are highly reactive. Despitethe importance of membrane stability, little is known about the effectof chemical reactivity on transport selectivity for these membranes.This Example uses size-selective, polysulfide-blocking membranes castfrom PIM-1, a polymer of intrinsic microporosity, as a model system forunderstanding the design rules needed to stabilize their performance asion-selective membranes for lithium-sulfur (Li—S) andlithium-polysulfide (Li—PS) EES devices.

Li—S and Li—PS EES devices are attractive technologies due to the highspecific capacity (1675 mAh g⁻¹) and low cost of sulfur. The reductionof sulfur to lithium sulfide proceeds through intermediates consistingof highly soluble lithium polysulfides—Li₂S_(n), where 4≤n≤8—that candiffuse across the cell and react with the anode, leading to thewell-known shuttle effect. This shuttling effect is known to decreasecell lifetime and efficiency. To address the polysulfide crossoverproblem, size-selective membranes based on polymers of intrinsicmicroporosity (PIMs) that block polysulfide crossover while allowingLi-ion transport may be used. PIMs are unique in that they havepermanent microporosity due to frustrated packing of polymer chains inthe solid state. This property makes PIMs both highly permeable and wellsuited as size-selective membranes because the pore size can becarefully chosen to block active-species crossover while allowinglithium ion transport. Despite these promising characteristics, littleis known about their chemical stability in EES devices or the impact ofpolymer reactivity on polymer structure and transport behavior.

During the operation of both Li—S and Li—PS EES devices, lithiumpolysulfides are in direct contact with the membrane. Lithiumpolysulfides are both nucleophilic and reducing to many organics withlow-lying LUMOs. It was hypothesized that electrophilic1,4-dicyanooxanthrene functionalities in PIM-1 might be prone tonucleophilic attack by Li₂S_(n), forming lithiated thioamides (FIG. 17).To that end, it was noted during post-mortem analysis of cycled Li—PSbatteries that PIM-1 membranes changed in color from bright yellow toorange, suggesting a chemical reaction had indeed taken place. Inaddition to the color change, membranes that were soaked in polysulfidesolution were subsequently insoluble in chloroform, while membranes thatwere soaked in solvent or electrolyte retained their chloroformsolubility. The product of that transformation was not immediatelyknown, nor was its impact on PIM-1's ion-selective transport ability.Thus, detailed chemical analysis of the reaction products was carriedout along the reaction trajectory using a variety of spectroscopicmethods—including in situ FT-IR and NMR spectroscopy—and local changesin PIM-1's pore chemistry were linked to changes in macroscale porearchitecture and related transport selectivity.

In order to understand the effect of chemical reactivity on theselectivity of PIM-1 membranes, long-term measurements of thepolysulfide blocking-ability of PIM-1 were conducted. During thesemeasurements, it was found that the effective diffusion coefficient(D_(eff)) of lithium polysulfides through PIM-1 membranes was notconstant; instead, it gradually increased over time. In order to testwhether this change in polysulfide-blocking was related to the proposedchemical reaction with lithium polysulfides or an unrelated membranedegradation mechanism, crossover rate as a function of Li₂S_(n)concentration in the electrolyte in contact with the membrane wassystematically investigated. These measurements were carried out byplacing a PIM-1 membrane of known thickness and area between twocompartments of electrolyte. One of these compartments (the retentate)contained an initial concentration (C₀) of Li₂S_(n), while the otherinitially contained none (the permeate). The concentration of Li₂S_(n)in the permeate compartment was then measured as a function of time, andD_(eff) of Li₂S_(n) through the membrane could be calculated from theslope of this plot (see below, FIGS. 21A-21C). For an ideal membranethat does not react with Li₂S_(n) or degrade otherwise, D_(eff) shouldbe small and should not change with time. It was observed that forC₀=0.20 M S as Li₂S₈, D_(eff) decreased from 1.2×10⁻⁹ to 6.4×10⁻¹⁰ cm²s⁻¹ during the first 15 h of the crossover experiment (FIGS. 18A-18B).This decrease was followed by a gradual increase to 7.9×10⁻¹⁰ cm² s⁻¹after 80 h. At higher C₀, the increase in D_(eff) with time was muchsharper. For instance, with C₀=0.50 M S, D_(eff) increased from6.3×10⁻¹⁰ to 2.0×10⁻⁹ cm² s⁻¹ after 50 h. For C₀=0.80 M and 1.0 M,D_(eff) had a lower initial value of 3.9×10⁻¹⁰ cm2 s⁻¹ followed by asharp increase to 2.0×10⁻⁹ cm² s⁻¹ after 18 h. At all concentrations ofsulfur, the membrane's polysulfide blocking-ability degraded, withfaster degradation at higher sulfur concentrations. This concentrationdependence implies that the degradation in membrane performance is dueto a chemical reaction rather than mechanical failure. This trend pointstowards a change in the pore structure that is caused by chemicalreactivity of the membrane.

To verify the proposed reactivity pathway with NMR and massspectrometry, model compound 1 was synthesized and allowed to react withexcess Li₂S₈ in 1:1 (v/v) THF-d₈:diglyme (FIGS. 19A-19D). In thepresence of 20 equiv. Li₂S₈, ¹H-NMR shows complete conversion of themodel compound into several different species of lithiated thioamides,all of which have one unreacted nitrile group (FIG. 19B, FIGS. 22A-24).This distribution of products was expected, as it is well known thatlithium polysulfides in solution exist as a variety of species withdifferent chain lengths. Negative-ion mode high-resolution electrosprayionization mass spectrometry (ESI-MS) provided further evidence for theconversion of the nitrile group in the model compound to a lithiatedthioamide. The most intense peak in the ESI-MS spectrum corresponded to[1+SH]⁻ (m/z obsd. 485.12, calc. 485.15), which forms from hydrolysis ofthe proposed species in the presence of adventitious water. Smallerpeaks corresponding to [1+Li₃S_(n)]⁻, where n=5 (m/z obsd. 633.04, calc.633.08), 6 (m/z obsd. 665.01, calc. 665.05), 7 (m/z obsd. 696.99, calc.697.03), and 8 (m/z obsd. 728.96, calc 729.00) were also observed,providing strong evidence for the conversion of 1 into lithiatedthioamide species (FIGS. 19C-19D, FIGS. 25-26).

Having established the reactivity of the nitrile groups of PIM-1 withthe aid of a model compound, it was sought to measure the extent andrate of this reaction with in situ FT-IR spectroscopy. A thin film ofPIM-1 was deposited on an ATR probe that was immersed in 1.0 M S asLi₂S₈ in electrolyte. The intensity of the nitrile stretch at 2239 cm⁻¹slowly decreased in intensity to 92% of its initial value after 22.5 h(FIGS. 20A-20C). Concomitantly, new stretches at 2221 cm⁻¹ and 1579 cm⁻¹appeared and grew in intensity. The stretch at 2221 cm⁻¹ is attributableto unreacted nitrile groups adjacent to the newly formed thioamide,while the stretch at 1579 cm⁻¹ is consistent with the thioamidefunctional group. After 22.5 h, the polysulfide solution was removed andreplaced with electrolyte, and the new peaks persisted, indicating thatthe chemical reaction is not reversible in the presence of electrolyte(FIG. 27). The time-scale for conversion of nitrile groups on PIM-1 tolithiated thioamides is similar to the time-scale of increased crossoverrates (FIG. 17), providing compelling evidence that the change inmembrane active-species blocking ability is due to its chemicalevolution. Further evidence for this hypothesis was provided byrepeating the in situ FT-IR experiment in the presence of 0.2 M S asLi₂S₈. As expected, the chemical reaction was slower, with the nitrilepeak only decreasing to 97% of its initial value after 22.5 h (FIG. 28).These experiments show that changes in membrane chemical reactivity aredirectly correlated to changes in membrane selectivity, with largerextents of conversion of nitrile to lithiated thioamide corresponding tolower membrane selectivity.

On the basis of model compound studies, in situ FT-IR, andconcentration-dependent crossover, it is clear that PIM-1 reacts withlithium polysulfides and that this reactivity correlates with decreasedactive-species blocking ability. In order to understand how the chemicalreactivity of PIM-1 leads to a change in active-species blocking, gasadsorption experiments were used to relate changes in polymer chemistryto changes in the pore structure of the membrane. PIM-1 membranes weresoaked in electrolyte or electrolyte containing 1.0 M S as Li₂S₈, washedthoroughly, and dried under vacuum at 120° C. for 19 h. Nitrogenadsorption isotherms were measured at 77 K, and BET surface areas andpore size distributions were calculated. Both isotherms werecharacterized by high nitrogen uptake at very low pressures that istypical of microporous materials, as well as pronounced hysteresis thatis commonly observed for PIMs. Unreacted PIM-1 membranes had a BETsurface area of 570 m² g⁻¹ and typical pore width of 9 Å, which isconsistent with reported values (FIG. 20C, FIGS. 29A-29C). In contrast,the reacted PIM-1 membranes had a BET surface area of 431 m² g⁻¹ alongwith larger pores with a typical width of 11 Å. These results suggestthat the reacted PIM-1 packs less efficiently than PIM-1 in the solidstate due to the presence of lithiated thioamide appendages. This changein polymer structure provides a reasonable explanation for the decreasedpolysulfide-blocking ability of PIM-1 after soaking in solutions oflithium polysulfides. In addition to changes in the dry polymerstructure after reaction with lithium polysulfides, it is also possiblethat the proposed reactivity effects the solvation of polymer chains,thus further altering the structure of the polymer in its swollen state.

Membranes capable of sustained blocking of active-species crossover arecritical for the implementation of next-generation EES devices.Unfortunately, relatively little is known about how proposed membranesevolve in the presence of highly reactive electrolytes. This Examplesystematically studied the chemical evolution of a promising membranematerial (PIM-1) in the presence of dissolved lithium polysulfides andfound that the nitrile groups on the polymer backbone react with lithiumpolysulfides to form lithiated thioamides. This change in chemicalstructure of the polymer leads to a change in the membrane's porearchitecture, causing a decrease in active-species blocking ability. Theinsights gained here highlight the importance of understanding theinterplay between chemical reactivity and membrane performance and pointthe way toward rational molecular design of selective membranes withimproved chemical stability and performance. The design rules exposed inthis Example indicate that PIM-1 membranes are suitable for use in cellswith relatively low concentrations of lithium polysulfides (i.e., Li−Sbatteries with a composite sulfur cathode). In the presence of higherpolysulfide concentrations, new strategies, including post-syntheticmodification to reduce reactivity and cross-linking may be useful forensuring long-term membrane selectivity.

Experimental. PIM-1 and model compound 1 were synthesized. Free-standingmembranes were prepared by drop-casting 12.5 mg mL⁻¹ solutions of thepolymer in chloroform into Teflon-coated wells under a crystallizationdish. After several hours of slow drying in air, the membranes weredried under vacuum and soaked in electrolyte for at least 6 hours.Crossover measurements were performed in a custom-purposed glass H-cellobtained from Adams & Chittenden Scientific Glass (Berkeley, Calif.)with an aperture diameter of 1.6 cm. Polysulfide concentration in thepermeate compartment was measured by cyclic voltammetry with a 1 mMdiameter glassy carbon working electrode and a lithium foilcounter/reference electrode. High-resolution electrospray ionizationmass spectrometry (ESI-MS) was performed in the negative ion mode on adilute sample of 1+20 equiv. Li₂S₈. Pore size distributions werecalculated from adsorption isotherms using the SAIEUS software packagewith a heterogeneous surfaces NLDFT model.

Figure Captions. FIG. 17. Proposed chemical reactivity between PIM-1 andlithium polysulfides, Li₂S_(n). Background: color change of PIM-1membrane after soaking in 2.5 M S as Li₂S₈ for 5 days.

FIG. 18A: Photograph of the H-cell used for crossover measurements andschematic depicting diffusion of Li₂S₈ across a membrane. FIG. 18B:Measured values of D_(eff) for Li₂S₈ across PIM-1 membranes as afunction of time for different initial concentrations of Li₂S₈.

FIG. 19A: Proposed reactivity of model compound 1. FIG. 19B: Aromaticregion of ¹H NMR before (top, red) and after (bottom, blue) the additionof 20 equiv. Li₂S₈ in 1:1 THF-d₈:diglyme. FIG. 19C: Observed(bottom/black) and calculated (top/green) ESI-MS spectra for [1+SH]⁻.FIG. 19D: Observed (bottom/black) and calculated (top/green) ESI-MSspectra for [1+Li₃S₅]⁻.

FIG. 20A: Time-evolution of FT-IR spectra for PIM-1 soaked in 1.0 M S asLi₂S₈. FIG. 20B: Time-evolution of peak intensities for peaks at 2239,2221, and 1579 cm⁻¹. FIG. 20C: Pore-size distribution of PIM-1 soaked inelectrolyte vs. soaked in electrolyte containing 1.0 M S as Li₂S₈ for 24h.

Materials. Diethylene glycol dimethyl ether (diglyme, anhydrous, 99.5%),1,2-dimethoxyethane (glyme, anhydrous, 99.5%), 4-tert-butylcatechol(98%), potassium carbonate, tetrafluoroterephthalonitrile (99%),tetrahydrofuran-d8 (99.5% atom D) and3,3,3′,3′-tetramethyl-1,1′-spirobisindane-5,5′,6,6′-tetraol (96%) wereobtained from Sigma-Aldrich. Lithium foil (99.9%, 1.5 mm thick), lithiumnitrate, lithium sulfide (99.9% metals basis), and sulfur (99.9995%metals basis) were obtained from Alfa Aesar. Lithiumbis(trifluoromethanesulfonimide) (LiTFSI) was obtained from 3M. Glassycarbon electrodes with 1 mm diameter were purchased from BAS Inc. (WestLafayette, Ind.) and polished before each experiment with 3-μm diamondpaste. N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) were takenfrom a JC Meyer solvent system. Chloroform (HPLC grade) was obtainedfrom EMD Millipore. All chemicals were used as received unless otherwisespecified. Lithium nitrate and LiTFSI were dried under vacuum for 16 hat 110 and 150° C., respectively. Diglyme was dried over 3 Å molecularsieves to <20 ppm water. Electrolyte refers to 0.50 M LiTFSI and 0.15 MLiNO₃ in diglyme. A stock solution of 2.50 M S as Li₂S₈ was prepared byadding sulfur (701 mg, 21.9 mmol S) and lithium sulfide (144 mg, 3.1mmol) to electrolyte (10 mL) at 60° C. The stock solution was stored at60° C. to prevent precipitation of polysulfides and was diluted asnecessary.

Instrumentation. Unless otherwise mentioned, all manipulations wereperformed in an argon glovebox with oxygen and water levels below 5 and1 ppm, respectively. NMR spectra were acquired on a Bruker Avance II 500MHz NMR spectrometer at 500 MHz for ¹H and 125.7 MHz for ¹³C. ¹H and ¹³Cchemical shifts were referenced with respect to residual solvent peaks(¹H (δ) chloroform-d₃ 7.26, ¹³C (δ) chloroform-d₃ 77.16, ¹H (δ) THF-d₈1.72 ppm). Polymer molecular weight was measured using size-exclusionchromatography with a Malvern Viscotek TDA 302 system calibrated with a99 kDa monodisperse polystyrene standard. Electrochemical experimentswere performed on a Bio-Logic VMP3 potentiostat. Cyclic voltammogramswere acquired with iR drop compensation by measuring the uncompensatedresistance with a 100 kHz impedance measurement and correcting for 85%of the expected drop. Water content measurements were performed on aMettler Toledo C20 Coulometric KF Titrator Karl-Fischer apparatus.Nitrogen adsorption measurements were performed at liquid nitrogentemperature (˜77 K) with a Micromeritics Tristar II 3020 adsorptionsystem. In situ FT-IR spectroscopy of PIM-1 in the presence of lithiumpolysulfides was performed with a Mettler Toledo ReactiR 15spectrometer. ESI-MS spectra were acquired with a Bruker microTOF-Qhigh-resolution mass-spectrometer.

Synthesis of PIM-1. PIM-1 with molecular weight 200 kg mol⁻¹ wassynthesized. Briefly, a mixture of anhydrous potassium carbonate (8.3 g,60 mmol), 3,3,3′,3′-tetramethyl-1,12-spirobisindane-5,5′,6,6′-tetrol(6.8 g, 20 mmol) and tetrafluoroterephthalonitrile (4.0 g, 20 mmol) indry DMF was stirred at 65° C. for 4 d. On cooling, the mixture was addedto water and the crude product collected by filtration. Repeatedprecipitations from a concentrated solution of polymer in chloroforminto methanol yielded 8.90 g (19.3 mmol, 97% yield) of the fluorescentyellow polymer (PIM-1).

Synthesis of model compound 1. Model compound 1 was synthesized.Briefly, an oven-dried 40 mL septum-capped vial was charged with a stirbar, 4-tert-butylcatechol (997 mg, 6 mmol),tetrafluoroterephthalonitrile (600 mg, 3 mmol), and dry DMF (13 mL). Themixture was stirred for several minutes to give a transparent orangesolution. Next, potassium carbonate (871 mg, 6.3 mmol) was added, andthe mixture was heated to 70° C. under nitrogen for 25 h. The resultingsuspension was added to 100 mL water, filtered, and rinsed with waterand acetone. Finally, the product was dried at reduced pressureovernight to yield 1.306 g (2.9 mmol, 96% yield) of 1 as a bright yellowpowder. ¹H (CDCl₃): δ 7.03 (dd, 2H, JHH=8.2, 2.2 Hz, ArH), 7.02 (d, 2H,JHH=2.1 Hz, ArH), 6.92 (d, 2H, JHH=8.2 Hz, ArH), 1.29 (s, 18H, CH3);¹³C{¹H} (CDCl₃): δ 149.9, 139.5, 139.3, 137.5, 122.5, 116.4, 114.4,94.4, 34.8, 31.3

Crossover measurement and analysis. A PIM-1 membrane of known thickness(typically 8-12 μm) was placed between two halves of an H-cell with anaperture diameter of 1.6 cm and sealed in place with a chemicallyresistant O-ring. One half of the H-cell (the retentate) was chargedwith 12 mL of Li₂S₈ in electrolyte, while the other half (the permeate)was charged with the same volume of electrolyte with no Li₂S₈. Bothcompartments were stirred to ensure homogeneity. Every 20-30 min, thestirring was stopped and the concentration was measuredelectrochemically by acquiring a CV at 100 mV s⁻¹ from 2.00 V to 3.00 Vvs. Li/Li⁻. The peak anodic current was related to polysulfideconcentration with a calibration curve (FIGS. 21A-C).

TABLE 1 Known concentration, calculated concentration from thecalibration curve, and the percent difference for all points on thecalibration curve Actual Conc. Calc. Conc. Difference (mM) (mM) (%)0.998 1.030 3.3 1.478 1.463 −1.0 1.992 1.952 −2.0 2.982 2.951 −1.0 3.9684.090 3.1 4.95 4.939 −0.2 5.929 5.843 −1.5 7.874 7.936 0.8 9.804 9.675−1.3 11.719 11.498 −1.9 15.034 14.832 −1.3 20.154 20.313 0.8 29.1929.357 0.6 37.893 38.651 2.0

Calculation of D_(eff) from crossover measurement. At any moment, theflux of active-species across the membrane (J, mol cm⁻² s⁻¹ can bedescribed with Fick's first law:

${J = {{D_{eff}\frac{\partial C}{\partial x}} = {D_{eff}\frac{{C_{retentate}(t)} - {C_{permeate}(t)}}{l}}}},$

where C is the concentration in mol cm ⁻³ and 1 is the membranethickness in cm. For short times, the differenceC_(retentate)(t)−C_(permeate)(t) does not change significantly from itsinitial value of C_(retentate)(t₀)−C_(permeate)(t₀)=C₀, and the flux isconstant with time:

$J_{t\text{\textasciitilde}0} = {D_{eff}{\frac{C_{0}}{l}.}}$

The concentration of active species in the permeate compartment can becalculated by integrating the flux of active species over time from 0 tot, multiplying by the membrane area, A, and dividing by the volume ofsolution in the permeate compartment:

${C_{permeate}(t)} = {\frac{A{\int_{0}^{t}{{J(t)}{dt}}}}{V_{permeate}} = {\frac{D_{eff}C_{0}A}{{lV}_{permeate}}{t.}}}$

By measuring active-species concentration in the retentate compartmentand plotting these values as a function of time, the effective diffusioncoefficient of the active-species through the membrane can bequantified.

Characterization of reacted model compound. Assignment of ¹H-NMR ofreacted model compound. Solutions of lithium polysulfides are maycontain numerous species. Therefore, a number of lithiated thioamidesmay result from the reaction of model compound 1 with Li₂S₈. Thealiphatic region of the ¹H NMR spectrum of 1+20 Li₂S₈ has one sharpsinglet at 1.28 ppm and three broad singlets at 1.21, 1.13, and 0.97 ppmwith relative integration of the sharp singlets to broad singlets of1:1. The sharp singlet, which is within 0.02 ppm of the unreactedcompound resonance, is attributable to tert-butyl groups on the oppositeside of the molecule from the reacted nitrile group (FIG. 22A, proton8). The broad peaks correspond to tert-butyl groups close to the reactednitrile, and can be assigned to two separate species: one where therotation around the C-CN bond is unhindered, and another where therotation is hindered. It was hypothesize that for lithiated thioamidescontaining more than 3 sulfur atoms (species B in FIG. 22A), the unboundterminal sulfur atom can chelate lithium along with the neighboringoxygen, thus hindering rotation about the C—CN bond. As a result, theprotons from the tert-butyl groups in the α and β conformers arechemically distinct, with peaks at 1.21 and 0.97 ppm. As temperature wasincreased to 55° C. (FIGS. 23A-23B), these peaks broadened as is typicalbefore coalescence, which supports this assignment. On the other hand,lithiated thioamides with fewer sulfur atoms cannot chelate lithium inthe same way, and so they have less hindered rotation about the C—CNbond, leading to one broad peak at 1.13 ppm for the signal averagebetween the α and β conformers. As expected, this peak did not broadenas temperature is increased. The multiplets from 7.1 to 6.9 ppm aresimilar in chemical shift to the multiplets in the unreacted modelcompound and can be assigned to protons 4, 5, and 6. This is furthersupported by the relative integration of the peaks, with the multipletsfrom 7.1 to 6.9 ppm having a relative integration of 3, equivalent tothe total integral from 6.8 to 6.1 ppm. The remaining peaks were readilyassigned on the basis of ¹H-COSY and integration data. H₃ protons wereassigned based on the absence of o-coupling and the absence of ¹H-COSYcross-peaks, with the upfield peak assigned to the more shielded protonof species B. Pairs of multiplets corresponding to H₁ and H₂ wereassigned based on ¹H-COSY cross-peaks, with the upfield pair assigned tospecies B and the more upfield of each pair of multiplets assigned toproton 2.

ESI-MS of reacted model compound. An 8 mM solution of 1 in 1:1diglyme:THF-d₈ was treated with 20 equivalents of Li₂S₈ in the samesolvent mixture. After 10 days mixing to ensure complete equilibration,the solution was diluted to 8×10⁻⁶ M in 1. To avoidcontamination/decomposition of the reacted model compound with water andoxygen, the syringe and capillary of the ESI-MS instrument were purgedwith dry, air-free THF immediately prior to analysis of the modelcompound with polysulfide. The ESI-MS was operated in negative mode withan injection rate of 5 μL/min.

Characterization of reacted PIM-1. FT-IR of PIM-1 in the presence oflithium polysulfides. PIM-1 was dropcast onto the polished silicon ATRprobe of the spectrometer from a 12.5 mg mL⁻¹ solution in chloroform,which was dipped into electrolyte blanketed under nitrogen. A stocksolution of Li₂S₈ in electrolyte was injected to yield a sulfurconcentration of 1.0 M or 0.2 M, as appropriate. The resulting solutionwas stirred under nitrogen and spectra were acquired every 5 min. Peakheights as shown in FIG. 27 were measured from a 2-point baseline.

Gas adsorption measurements of PIM-1. PIM-1 was soaked in electrolyte orelectrolyte containing 1.0 M S as Li₂S₈ for 24 h, followed by washingwith and soaking in diglyme for a total of 26 h. Finally, the membraneswere washed with glyme, dried under vacuum at room temperature for 70 h,and dried under vacuum at 120° C. for 19 h before measurement.

Figure Captions. FIG. 21A: Calibration plot of log(current) vs.log(concentration) with the linear regression. FIG. 21B: Residuals fromFIG. 21A, showing that the deviations from the fit are random. FIG. 21C:The calibration plot of FIG. 21A on linear axes.

FIG. 22A: Proposed chemical structure of model compound 1 after reactionwith lithium polysulfides, FIG. 22B: Aromatic region of the ¹H-NMR ofmodel compound 1 before (red, top) and after (blue, bottom) reactionwith 20 equiv. Li₂S₈ with peak assignments. FIG. 22C: Aliphatic regionof the ¹H-NMR of model compound 1 before (red, top) and after (blue,bottom) reaction with 20 equiv. Li₂S₈ with peak assignments.

FIG. 23A: Variable temperature ¹H-NMR of model compound 1+20 equiv.Li₂S₈ at 25, 45, and 55° C. for the aromatic region of the spectrum.FIG. 23B: Variable temperature ¹H-NMR of model compound 1+20 equiv.Li₂S₈ at 25, 45, and 55° C. for the aliphatic region of the spectrum.

FIG. 24: ¹H-COSY of model compound 1+20 equiv. Li₂S₈.

FIG. 25: ESI-MS showing the most intense peak assigned to [M+SH]⁻.

FIG. 26: Lower intensity region from FIG. 25 highlighting peaksattributed to both nitrile groups of the model compound reacting withpolysulfide. Isotopic distributions for all assigned peaks are similarlywell matched to those displayed above.

FIG. 27: FT-IR of PIM-1 after soaking in 1.0 M S as Li₂S₈ in electrolytefor 22.5 h (black) and after replacing the Li₂S₈ solution with freshelectrolyte and soaking for an additional 8.5 h (violet).

FIG. 28: Normalized intensity of the nitrile stretch at 2239 cm⁻¹ ofPIM-1 in the presence of 0.2 M (black) and 1.0 M (red) S as Li₂S₈.

FIG. 29A: Adsorption (filled circles) and desorption (hollow circles)isotherms. FIG. 29B: BET surface area analysis for PIM-1 soaked inelectrolyte (blue) and electrolyte containing 1.0 M S as Li₂S₈ (red).FIG. 29C: Simulated NLDFT adsorption isotherms (lines) with experimentalisotherms (points) for PIM-1 soaked in electrolyte (blue) andelectrolyte containing 1.0 M S as Li₂ S₈ (red).

Example 3 Redox-Switchable Microporous Polymer Membranes that Extend theCycle-Life of Lithium-Sulfur Batteries

In biological systems, selective transport of ions across membranes isachieved by transmembrane proteins. Ion flux is subject to strictregulation and the cell's environment plays a dominant role.Perturbations to that environment—whether physical, chemical, orelectrical—are met with an adaptive response, which is tied to changesin the proteins' transport functions. This example applies this conceptof adaptive transport across ion-selective membranes to improve thecycle-life of lithium-sulfur (Li—S) batteries. Li—S batteries areinherently unstable when soluble polysulfides in the cathode crossoverthe membrane and react with the lithium-metal anode. This Example showsthat certain types of redox-switchable microporous polymer membranes canbe made to enhance their selectivity for the battery's working ion whenthey encounter the battery's endogenous polysulfides and that thesefunctions are sustained. The origins and implications of this behaviorare explored in detail and point to new opportunities in responsivemembranes.

Membrane separators play a critical role in many battery technologies,where they serve to electronically isolate the anode from the cathodewhile allowing the working ion to diffuse between them. For batterychemistries that involve dissolved, dispersed, or suspended activematerials, membrane separators must also prevent active-materialcrossover; failure to do so leads to low round-trip energy efficiencyand in some cases unacceptable capacity fade. This is particularlyproblematic in lithium-sulfur (Li—S) batteries, where inefficiencies andinstabilities arise when soluble polysulfides—intermediates in theelectrochemical interconversion of S₈ and Li₂S—crossover and incur ashuttling current or otherwise react with the lithium-metal anode.

This Example shows that these shortcomings are alleviated in the Li—Sbattery when its membrane separator is configured rationally fromredox-switchable polymers of intrinsic microporosity (PIMs). Advantagesmay be achieved for adaptive transport selectivity for the working ion,which productively leverages the reducing environment of the sulfurcathode to chemically transform a charge-neutral size-selective PIMmembrane into a lithiated PIM membrane with enhancedpolysulfide-rejecting properties. The design of these new adaptive PIMmembranes was navigated computationally, where putative monomer segmentswere screened for their susceptibility to reduction by polysulfides(i.e., an electron affinity above 2.5 V vs Li/Li⁺). Those predictionswere experimentally validated to demonstrate that progressive reductionand lithiation of the PIM membrane by polysulfides slows polysulfidediffusive permeability from—providing an impressive improvement overnon-selective Celgard separators—without impacting the intrinsic ionicconductivity for solvated lithium ions. This Example also shows that byblocking polysulfide crossover, cycle-life of Li—S cells markedlyimprove—most notably in the absence of lithium-anode protectingadditives. The stability of the lithium anode under these conditions isunprecedented, and highlights the unexpected and exciting newopportunities afforded by responsive redox-active polymers andultimately adaptive membranes in battery technology development.

Design strategies for a membrane that offers selectivity may make use oftwo mechanisms: size-sieving and electrostatic charge-blocking. Guidedby theoretical calculations to target a specific turn-on voltage forelectrostatic charge-blocking, redox active moieties capable of in situreduction were selected. These were then incorporated into a polymer ofintrinsic microporosity (PIM-7), a class of polymer well known for sizesieving by virtue of its permanent microporous architecture. Duringbattery operation, these redox-active groups become charged, leading tocharge selectivity. Using lithium-sulfur batteries as a model chemistryto probe in operando activated membrane selectivity, the PIM-7 platformwas tested in lithium-sulfur cells for its ability to block polysulfidecrossover and shuttling at the anode. The redox activity of the polymerat 3.0 V vs. Li/Li⁺ was confirmed via electrochemical reduction (CV),and chemical reduction of PIM-7 by Li₂S was monitored with UV-Vis.Synergistic size-sieving and charge repulsion of polysulfide was evidentin the crossover behavior and lithium-sulfur cell performance where thehybrid charge and size selective PIM-7 membrane vastly outperforms thatof a commercial separator as well as a PIM membrane capable of sizeexclusion alone.

Given the synthetic versatility of the PIM platform, many monomerscontaining redox active units are available. To help guide theselection, theoretical calculations were used to facilitate determininga monomer with a compatible reduction potential for a Li—S battery. Froma library of model compounds it was found that a class of phenazinecontaining monomers had redox potentials well suited to be reduced inthe Li—S battery (FIG. 31). In particular, PIM-7 was selected due to itsideal combination of redox potential (calculated: 2.90 V vs. Li/Li⁺),synthetic accessibility and membrane processability. PIM-7 wassynthesized (80 kg mol⁻¹) via step growth polymerization between aphenazyl-based bis(dichloro) monomer and the seminal spirobisindanebis(catechol). In order to utilize thinner membranes for higher ion fluxwithout sacrificing mechanical stability, PIM-7 membranes were cast ontoa Celgard support using wire wound rod processing. This method afforded2 μm thick uniform coatings of PIM-7 as evidenced by cross-sectionalSEM. PIM-7 has a reported BET specific surface area (680 m² g⁻¹) and apore size of 0.70 nm, which is ideal for selective transport of LiTFSIand PS blocking. PIM-7 has a built in redox-switchable moiety that, inaccordance with calculations, is capable of providing in operando chargeblocking, while also possessing features that result in size sieving.

In order to experimentally validate the calculated reduction potentialof PIM-7, cyclic voltammetry (CV) was carried out on the polymer dropcast onto a glassy carbon working electrode (FIG. 32A). PIM-7 showed tworeversible reduction peaks at E_(1/2)=3.05 and 2.85 V vs. Li/Li⁺, whichcan be attributed to the reduction of the phenazine group to the radicalanion followed by the reduction to the dianionic species. To furtherverify this result, PIM-7 was chemically reduced via reaction with Li₂S,where reduction is indicated via a shift to lower energies (from 440 to340 nm) in the UV-Vis (FIG. 32B). These results indicate that the PIM-7membrane will be negatively charged in the reducing environment of theLi—S battery, both chemically by components of the catholyte, andelectrochemically by contact with the cathode current collector. Thisredox switchable design is advantageous because by casting the polymerin its neutral state, common pitfalls associated with the processing ofcharged materials, such as solubility and mechanical stability, areavoided. The membrane can then be redox activated once in the cell toafford polysulfide blocking via charge repulsion.

After confirming the redox switching behavior of the PIM-7 membrane, theeffect that embedded charged moieties had on the ability to mitigate thepolysulfide shuttle was examined. Active species crossover measurementswere obtained using supported PIM membranes of a known area andthickness placed between two chambers of an H-cell. The H-cell isconfigured with dissolved PS (0.8 M S as Li₂ S₈ in diglyme containing0.50 M LiTFSI and 0.15 M LiNO₃) on the retentate side and PS-freeelectrolyte on the permeate side (FIG. 33A, inset). The evolution of PSon the permeate side is then monitored over time using cyclicvoltammetry, as the peak current can be directly related to theconcentration via a calibration curve. Using an initial rateapproximation, the diffusion coefficient of PS was calculated to be6.2×10⁻⁸ cm²/s for Celgard, 7.1×10⁻⁹ cm²/s for PIM-1, and 1.5×10⁻⁹ cm²/sfor PIM-7. The 2 μm PIM-1 blocking layer provides approximately a 5 foldreduction in the crossover of PS and the PIM-7 enhances this blockingability by 40 fold (FIG. 33A). Additionally, if supported PIM-7membranes are systematically reduced prior to conducting crossovermeasurements (0, 12, and 24 h, respectively) enhanced blocking abilityis observed for membranes that have been in a reducing environment forlonger times (FIG. 33B). This effect may be attributable to slowdiffusion of polysulfide through the membrane and the positive feedbackloop of reduced membrane further retarding the migration of polysulfide.Pre-reduction treatment past 24 h does not appear to enhance blocking.Furthermore, the supported PIM-7 also demonstrates a crossover rate thatis stable over an extended period (2 d) without showing any degradation,unlike the PIM-1 which reacts with polysulfides over time and results ingreater crossover. Likewise, the change in membrane ionic conductivityupon soaking for 0, 12, and 24 h in polysulfides was measured and it wasfound that, unlike the PIM-1, the impedance remained constant. Thissuggests that an additional selectivity mechanism may be available insitu without sacrificing conductivity. These crossover measurementssupport the idea that a hybrid membrane capable of both charge blockingand size sieving has been achieved without sacrificing ion flux, andthat the design strategy described herein is advantageous for thelong-term stability of the membrane.

In order to highlight the utility of a hybrid size and charge-selectivemembrane, the performance of PIM-7 membranes was tested in energy denseLi—S batteries. Li—S cells were constructed using Swagelok cellscomprised of a lithium anode and polysulfide catholyte (0.5 M S as Li₂S₈in diglyme containing 0.50 M LiTFSI and 5 wt % Ketjenblack) separated bya supported PIM-7 membrane. The standard anode protecting additive LiNO₃was intentionally omitted from the catholyte formulation in order tomore directly assess the influence of each membrane in mitigating thepolysulfide shuttle. Analogous cells with Celgard and PIM-1 membraneswere constructed for comparison with all cells cycling at a rate of C/4for 50 cycles. The Li—S cells containing the Celgard separator initiallyexhibit capacities comparable to that of the cells containing PIM-1 andPIM-7 membranes, however, after 5-10 cycles the cells failed due tofailure to reach the charging cutoff voltage as a result of unrestrainedpolysulfide shuttling ultimately leading to cell failure was observed.Cells containing the PIM-1 membrane were able to sustain capacities of300 mAh/g over 50 cycles, comparable to the results demonstrated in theabove Examples. In accordance with the superior blocking ability of thePIM-7 seen in the crossover measurements, cells constructed with thePIM-7 membrane displayed vastly improved performance over that of theCelgard and PIM-1. The PIM-7 membrane cells were able to sustaincapacities of 900 mAh/g over 50 cycles (FIG. 34). Additionally the cellsconstructed with PIM-7 displayed higher Coulombic efficiencies ascompared with both Celgard and PIM-1. The rate capability of the PIM-7membrane was assessed by cycling at C/4, C/2 and C in sequence. Even atthe high 1 C rate, the cells had capacities indicating that the ionicconductivity of these membranes is sufficient for higher chargedensities. These results represent an improvement over related Li—S workand demonstrate the potential for a hybrid membrane material to improveselectivity via a secondary mechanism.

The design strategies for membrane materials for electrochemical deviceshave reached an impasse: it is difficult to control the selectivetransport properties of the membrane without negatively impactingmembrane conductivity. To address this bottleneck a membrane materialcapable of selectivity by virtue of two mechanisms has been designed,synthesized, and processed. The key design feature is the incorporationof a redox active moiety, chosen via a predictive materials genome to bereduced in situ in the Li—S battery, into the backbone of the PIM. Thishybrid approach provides a charge repulsion blocking mechanism whilestill preserving the structural features that lead to size-sieving. Thisresults in a marked improvement in crossover performance of the PIM-7when compared to the PIM-1 membrane, without dramatically sacrificingmembrane impedance. Additionally, the PIM-7 membranes displayed betterperformance in capacity, cycle life and Coulombic efficiencies whenutilized in Li—S batteries. Membranes can now be tuned for pore size aswell as pore chemistry. Using the predictive genome, it isstraightforward to use this hybrid approach to selectivity to tailorboth the size and charge of the PIM pores to suit the transport propertyneeds of a multitude of energy storage devices.

Figure captions. FIG. 30: Hybrid membrane design for achieving both sizeand charge selectivity. This Example introduces a platform based onpolymers of intrinsic microporosity that demonstrates selectivity basedon charge exclusion can be achieved by incorporation of an in situactivated redox switch into the polymer backbone as well as by sizeselectivity imparted by virtue of the micropore architecture.

FIG. 31: Library of model compounds and calculated reduction potentialsused to guide the selection of redox switches for membraneincorporation. PIM-7 (highlighted) was selected for its idealcombination of reduction potential, synthetic accessibility and membraneprocessability.

FIG. 32A: Cyclic voltammogram of the PIM-7 polymer showing tworeversible reductions at E_(1/2)=3.05 and 2.85 V vs. Li/Li⁻. FIG. 32B:UV-Vis spectra of the PIM-7 polymer before and after chemical reductionwith Li₂ S.

FIG. 33A: Time-evolution of the concentration of PS in the permeate(left) of H-cells configured with either a Celgard (grey), PIM-1 (green)or a PIM-7 (purple) membrane. FIG. 33B: Time-evolution of theconcentration of PS in the permeate (left) of H-cells configured withPIM-7 membranes pre reduced for 0 h (circle), 12 h (square) or 24 h(triangle). The retentate was charged with an initial concentration of0.8 M S as Li₂S₈ in diglyme containing 0.50 M LiTFSI and 0.15 M LiNO₃.The concentration of PS in the permeate was determinedelectrochemically.

FIG. 34: Discharge capacity vs. cycle number at C/4 rate for cellsconstructed with supported PIM-1 and PIM-7.

Methods. Materials: Diglyme (G2),3,3,3′,3′-tetramethyl-1,1′-spirobiindane-5,5′,6,6′-tetraol andtetrafluoroterephthalonitrile were purchased from Sigma Aldrich. Sulfur(Puratronic, 99.9995% (metals basis)), lithium sulfide (99.9% (metalsbasis)), and lithium metal were purchased from Alfa Aesar. Lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI) was purchased from 3M.Celgard 2535 membrane was purchased from MTI Corporation. KetjenblackEC-600JD was purchased from AkzoNobel.

Membrane preparation: Supported membranes of PIM-1 and PIM-7 wereprepared. Electrolyte and catholyte preparation: The supportingelectrolyte formulation for all battery cycling and conductivitymeasurements was 0.50 M LiTFSI. LiNO₃ was added to the electrolyte onlyfor the crossover experiments detailed below. LiTFSI was dried for 16 hunder vacuum at 150° C. LiNO₃ was dried for 16 h under vacuum at 110° C.Diglyme was tested for peroxides prior to use; if any were measured, itwas stirred with alumina, filtered, and sparged with argon. Diglyme wasdried with activated 3 Å molecular sieves until it measured <20 ppm H₂O.Electrolyte was tested for water content and confirmed to contain <30ppm water before use. Solutions of Li₂S₈ (2.50 mol S L⁻¹ in electrolyte)were prepared by mixing Li₂S (0.287 g, 6.25 mmol), sulfur (1.40 g, 5.47mmol), and 20 mL of electrolyte and heating at 60° C. until all solidswere dissolved. Li₂S₈ solutions were kept at 60° C. in order to preventprecipitation of insoluble species and cooled to room temperature priorto use. Cathode slurry were prepared with 5% w/w Ketjenblack and 0.5 MLi₂S₈ solution and sonicated for 30 min at 50° C.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, one of skill in the art will appreciate that certainchanges and modifications may be practiced within the scope of theappended claims. In addition, each reference provided herein isincorporated by reference in its entirety to the same extent as if eachreference was individually incorporated by reference. Where a conflictexists between the instant application and a reference provided herein,the instant application shall dominate.

1. A modified polymer of intrinsic microporosity comprising a polymer ofintrinsic microporosity having a plurality of repeat units, wherein atleast one of the repeat units includes one or more negative charges, orwherein the polymer of intrinsic microporosity is crosslinked, or both.2. The modified polymer of intrinsic microporosity of claim 1, whereinat least one of the repeat units includes a negatively charged nitrogensite, a negatively charged oxygen site, a negatively charged sulfursite, a negatively charged carbon site, or any combination thereof. 3.The modified polymer of intrinsic microporosity of claim 1, wherein atleast one of the repeat units includes one or more charged moietiesselected from the group consisting of:

where subscript m and subscript o are independently integers selectedfrom 1 to
 8. 4. The modified polymer of intrinsic microporosity of claim1, wherein at least one the repeat units have a structure selected fromthe group consisting of:

where subscript m and subscript o are independently integers selectedfrom 1 to 8, and wherein subscript n is an integer selected from 10 to1000.
 5. The modified polymer of intrinsic microporosity of claim 1,wherein at least one repeat unit is crosslinked with a non-adjacentrepeat unit.
 6. The modified polymer of intrinsic microporosity of claim1, wherein at least one repeat unit is crosslinked with a non-adjacentrepeat unit by a crosslinker selected from the group consisting of2,6-bis(4-azidobenzylidene)cyclohexanone,2,6-bis(4-azidobenzylidene)-4-methylcyclohexanone,2,6-bis(4-azidobenzylidene)-4-ethylcyclohexanone, 4-azidophenylsulfone,and any combination of these.
 7. The modified polymer of intrinsicmicroporosity of claim 1, wherein a surface area of the polymer ofintrinsic microporosity is at least 300 m²/g.
 8. The modified polymer ofintrinsic microporosity of claim 1, further comprising a supportmembrane in contact with the polymer of intrinsic microporosity.
 9. Amethod, comprising: forming a reaction mixture comprising a polymer ofintrinsic microporosity and a reducing agent, or a nucleophile, or acrosslinking agent, or any combination thereof under conditionssufficient to form a modified polymer of intrinsic microporosity;wherein the modified polymer of intrinsic microporosity comprises aplurality of repeat units; and wherein at least one of the repeat unitsincludes one or more negative charges, or wherein the modified polymerof intrinsic microporosity is crosslinked, or both.
 10. The method ofclaim 9, wherein reaction between the polymer of intrinsic microporosityand the reducing agent or the nucleophile generates the one or morenegative charges.
 11. The method of claim 10, wherein the reducing agentcomprises one or more of an alkali metal polysulfide, Li₂S_(m) wheresubscript m is an integer selected from 2 to 100, an alkali metalsulfide, ammonium sulfide, an alkali metal hydrogen sulfide, an alkalimetal, a metallocene, an alkali metal naphthalenide, an inorganicreducing agent having an oxidation potential at or below 0.0 V vs. astandard hydrogen electrode (SHE), and an organic reducing agent havingan oxidation potential at or below 0.0 V vs. SHE.
 12. The method ofclaim 10, wherein the nucleophile comprises one or more of an alkalimetal polysulfide, Li₂S_(m) where subscript m is an integer selectedfrom 2 to 100, an alkali metal sulfide, ammonium sulfide, an alkalimetal hydrogen sulfide, an alkali metal alkylsulfide, an alkali metalarylsulfide, P₂S₅, and an alkali metal sulfite.
 13. The method of claim9, wherein reaction between the polymer of intrinsic microporosity andthe crosslinking agent induces crosslinking of the polymer of intrinsicmicroporosity by generating one or more covalent bonds between a firstrepeat unit and a second repeat unit that is not adjacent to the firstrepeat unit.
 14. The method of claim 13, wherein the crosslinking agentcomprises one or more of 2,6-bis(4-azidobenzylidene)cyclohexanone,2,6-bis(4-azidobenzylidene)-4-methylcyclohexanone,2,6-bis(4-azidobenzylidene)-4-ethylcyclohexanone, 4-azidophenylsulfone,oxygen, and any combination of these.
 15. The method of claim 9, whereinforming a reaction mixture comprising a polymer of intrinsicmicroporosity and a crosslinking agent comprises inducing a crosslinkingreaction of the polymer of intrinsic microporosity by exposure of thepolymer of intrinsic microporosity to one or more of ultravioletradiation, microwave radiation, and heat.
 16. (canceled)
 17. (canceled)18. An electrochemical cell comprising: an anode; an anode electrolytein contact with the anode; a separator in contact with the anodeelectrolyte, wherein the separator comprises a polymer of intrinsicmicroporosity; a cathode electrolyte in contact with the separator; anda cathode in contact with the cathode electrolyte.
 19. (canceled) 20.The electrochemical cell of claim 18, wherein the separator furthercomprises a support membrane in contact with the polymer of intrinsicmicroporosity.
 21. The electrochemical cell of claim 20, wherein thesupport membrane comprises a polymer selected from the group consistingof: polyethylene, polyethylene copolymers, polypropylene, polypropylenecopolymers, polyacrylonitrile, polyacrylonitrile copolymers,poly(vinylidene fluoride), poly(tetrafluoroethylene), poly(vinylchloride), poly(vinylchloride) copolymers, poly(hexafluoropropylene),poly(hexafluoropropylene) copolymers, polyaramide, any combinationthereof, and any copolymers thereof.
 22. The electrochemical cell ofclaim 20, wherein the support membrane has a melting temperature, andwherein exposing the support membrane to a temperature exceeding themelting temperature causes at least a portion of the support membrane tomelt and close pores within the separator.
 23. (canceled)
 24. The methodof claim 9, further comprising: contacting a first side of a separatorwith a first ionic solution, wherein the separator comprises themodified polymer of intrinsic microporosity, and wherein the first ionicsolution comprises a first ionic species; contacting a second side ofthe separator with a second ionic solution, wherein the second ionicsolution comprises a second ionic species; and transporting the firstionic species between the first ionic solution and the second ionicsolution through the separator; wherein the separator provides a sizeselective restriction on transport of the second ionic species from thesecond ionic solution to the first ionic solution through the separator.25. (canceled)
 26. (canceled)